MacCready was born in 2265. He has served as the mayor of Little Lamplight for three years (since he was ten) when he was chosen by the other residents due to his smarts, toughness and the fact that he doesn't take guff from anyone. He has no fear of adults and is a perfect representative. He is belligerent, small (even for his age), very distrustful of adults and willing to blow someone's head off to defend his young charges, but MacCready is also very loyal to those he considers friends.
MacCready came into office in 2274 shortly after the five-minute rule of Princess. According to Sammy, when Princess demanded the title of mayor to be changed to princess, MacCready punched her in the nose and said that the children of Lamplight needed a leader, not a princess. He was then elected to the position of mayor.
During the decade between 2277 and 2287, MacCready ended up marrying a woman named Lucy and had a son with her named Duncan. When he first met Lucy, he lied to her about his occupation, telling her he was a soldier and concealing the fact he was a hired gun. He never told her the truth. At an unknown point in time, feral ghouls killed Lucy while MacCready fled with Duncan. Some time later, Duncan contracted an unknown disease, prompting MacCready to travel to the Commonwealth to find a cure.
He was employed by the Gunners for some time, but left and has ever since been hounded by them for continuing to take jobs in their territory. He is still skilled with rifles and can make precise head shots. When the Sole Survivor meets him in The Third Rail, he's talking with two Gunners. After they leave, he can be hired as a companion for 250 caps (can be persuaded down to 200 or 100, depending on the size of the player's caps purse). This will be given back to the Sole Survivor after completing one of the parts of his quest. After becoming good friends with him and completing his quest; he will give the Killshot perk, which grants a 20% accuracy boost for head shots in V.A.T.S.
Picking Up the Trail/Rescue from Paradise: When first met, MacCready will threaten to shoot unless leaving the Little Lamplight cavern entrance. The Lone Wanderer has to pass a difficult speech check, use the Child at Heart perk, or complete the Rescue from Paradise quest in order to gain entrance to Little Lamplight. After gaining entrance to Little Lamplight, the Lone Wanderer will have a new dialogue path with MacCready which reveals the entrance to Vault 87 through Murder Pass.
Fungus Deal: After speaking with either Eclair or Lucy, the Lone Wanderer can convince MacCready to enter a trade agreement: swapping either strange meat (to Eclair) or Buffout (to Lucy) for cave fungus.
Those!: The Lone Wanderer can convince MacCready (using the Child at Heart perk) to accept Bryan Wilks into Little Lamplight.
Speaking to MacCready in quick succession too many times will raise his ire. Eventually he'll refuse to talk and will tell the Lone Wanderer to leave Little Lamplight. Although one can still talk to him later on, they must wait twenty-four hours to end the silent treatment.
MacCready for Action: This unmarked quest consists of convincing MacCready to join the Sole Survivor.
Long Road Ahead: MacCready asks the Sole Survivor to help him take care of the Gunners who were harassing him and later, to help him find a cure for his son. NOTE: MacCready's affection gains will cap at 499 until the Gunners portion of this quest is completed.
MacCready can join the Sole Survivor as a companion for a price of 250 caps. He can be bargained down to 200 or 100 caps, depending on the size of the Sole Survivor's caps purse, though the latter persuasion attempt is very demanding. This will be returned to the Sole Survivor after completing an early part of his quest.
If the player character reaches highest Affinity with MacCready he will give the perk Killshot, which boosts hit percentage for heads in VATS by 20%.
He will occasionally give a bit of ammo of random types, including fusion cores.
He will steal when commanded to do so by the Sole Survivor. He is able to steal items undetected right in front of people.
Commanding MacCready to enter power armor can prove difficult, especially in tight spaces. This is due to his AI being unable to find a path to the back of the armor frame.
MacCready likes when the Sole Survivor steals things.
MacCready likes it if the Sole Survivor picks the lock to a high level safe that is owned.
MacCready likes using most special dialogue options that require Charisma, only if one lies, threatens or asks for more money though.
Caution: He will dislike when opening the cell door after Lorenzo's death, assuming one freed him. This can be avoided by having MacCready wait outside the cell area or dismissing him, and then waiting for him to leave via the elevator.
When he is the companion, upon talking to him he will give some random ammo, ranging from 10mm rounds to a fusion core.
Every time he enters power armor, he will unequip its helmet and place it into his inventory. One can force him to equip the helmet again through trading, after which he won't try to unequip the headpiece until he enters a new set of power armor.
Once idolized, MacCready gives the player a wooden soldier toy, a gift from his wife, Lucy. Lucy made the toy soldier for him after he told her that he was a soldier when they first met.
Whitechapel Charlie's quest is a great way to farm likes from him. Starting the quest, checks for more caps, picking owned locks on the warehouse, and stealing in the warehouses all boost his affection.
As an adult, MacCready makes attempts to curb the cursing habit he had as a child. Occasionally, he will stop himself mid-sentence to correct his vulgar language. He does this because he made a promise to his son, Duncan, that he would clean up his behavior and "be a better person."
During Duty or Dishonor, MacCready hates it when agreeing not to kill the ghouls and/or to let Initiate Clarke go free. This is presumably related to the fact that ferals killed Lucy.
He appears to dislike getting wet, as he makes multiple negative comments about it throughout the game.
MacCready dislikes traveling at night, claiming "we could be walking into an ambush and never see it coming."
MacCready is a smoker. He mentions needing a cigarette in one of his ambient dialogue options. He will also light one when idle.
Robert has killed "for a drink" before. It is not clarified what kind of drink or what the circumstances were. This could conceivably mean that MacCready is hinting at having done anything from committing murder for booze to killing for life-sustaining water.
If taken to an abandoned nursery area in Trinity Church, he'll tear up, but pretend he has something in his eye.
In the gym area/basketball court of the Boston mayoral shelter, MacCready will comment: "We used to have a hoop setup in Little Lamplight until Eclair broke the darn thing."
Upon death, his last words will be "No... please... no."
When underground, MacCready randomly will shout "Tunnel Snakes Rule!," which is a reference to the Tunnel Snakes in Fallout 3, which MacCready appeared in.
When visiting Diamond City, MacCready will comment: "Haven't been to Diamond City in years but I'll tell you... nothing's changed." When brought to the Dugout Inn, Vadim Bobrov will greet him enthusiastically and say: "MacCready! Is good to see you, tovarisch. How is Lucy? She still as beautiful as I remember?" MacCready will correct him by saying: "No... she didn’t make it, Vadim." This all suggests that MacCready and Lucy traveled to the Commonwealth together and had been to Diamond City sometime before she passed and before Duncan got sick. Duncan may not have even been born, yet, as Vadim does not mention him.
When entering Vault 81, MacCready comments on how he lived near a vault (Vault 87) and that it was dangerous. His tone of voice suggests he's distrustful of vaults.
If MacCready is a companion in Special Delivery, when speaking to Jack Cabot he will imply that he believes in the extraterrestrial, and that it freaks him out. This could be a reference to Zeta, an alien mothership in Fallout 3. This may also hint that he is fascinated with space and/or science fiction, considering in Nuka-World's quest Trip to the Stars, he likes riding in the UFO ride.
When interacting with magazines, MacCready might comment on how he loves comic books.
When exploring the Super Duper Mart, MacCready might lament: "Of course it had to be the cookie aisle that got crushed."
After sleeping with MacCready, he may say: "I never knew you could do that with mutfruit..." This is a reference to the sexual act regarding grapefruit.
When taken to WRVR broadcast station, MacCready mentions the Capital Wasteland's radio GNR and Three Dog, saying "I miss the Capital Wasteland Radio... Three Dog was a hell of a DJ."
If the player character attempts to steal from the VIP room in The Third Rail, MacCready will leave their company and become hostile, prompting an interface for the player to send MacCready to a settlement, similar to Preston Garvey if the player kills a settler.
If the player commands him to steal an item, or does anything to cause hostility while in Goodneighbor, MacCready will leave the Sole Survivor's company and become hostile, prompting an interface to send MacCready to a settlement.
If taken to the Boston Public Library, he will state that lions scare children in a horrified voice, a reference to his childhood character in Fallout 3.
If MacCready is the player character's companion during the first conversation with Magnolia in The Third Rail, he states that he is a fan of rock and roll.
Upon reaching Mahkra Fishpacking MacCready will mention that fish gives him terrible flatulence.
MacCready hints a case of severe depression throughout Fallout 4, such as when he tells the Sole Survivor that he thought it would've been better if he and Duncan died with Lucy, and upon romancing him, he mentions that they have saved him from himself. This could also hint he's had thoughts of suicide.
In concept art, MacCready is misspelled as McReady.
In idle dialogue or when selecting "Thoughts?" when speaking to him, he might say "I gotta be honest with you, I'd feel more comfortable with a rocky ceiling above my head", a reference to Little Lamplight.
Some of MacCready's dialogue hints that he gets motion sickness, further evidenced by his commentary by the sea, or upon boarding a Vertibird.
MacCready has the tendency to break out idioms and puns, upon commentary depending on location.
"R.J. MacReady" is the protagonist from the 1982 John Carpenter horror film, The Thing, based on the novella "Who Goes There", by John Campbell. In the 1982 movie, MacReady is played by Kurt Russell. MacCready, like his Fallout 3 counterpart, is a no-nonsense, resourceful survivor who rises to a leadership position among his peers.
The quote "Let's be careful. No human being would possibly pile books this way." is referring to a line said by Bill Murray's character, Peter Venkman, in the 1984 movie Ghostbusters.
The quote "I never knew you could do that with mutfruit." is in reference to being "grapefruited" as instructed by Auntie Angel's demonstration seen here: 
The quote "School's out forever, right?" is referring to the song "School's Out" by Alice Cooper. The slight chanting further endorses this.
pc Mayor MacCready is prone to disappearing. He tends to wander into obscure and dark corners. He may appear near the front gate if going outside and fast travel back to town, or standing on the platform where he first stood when he let the Lone Wanderer into town. One can move to his current position with the console command player.moveto 0000313b. [verified]
pc Sometimes MacCready will not give the option to open the gate, even if one is at that point and have followed the advice outlined in this wiki. One way to make sure that this does not befall the player is to click all of the grayed out speech options when speaking to MacCready. The dialog option to open the gate comes after a line that is often grayed/dimmed. [verified]
pc It's possible for MacCready to not open the gate after being instructed to do so. He will run to it, and climb up to the catwalk but will not activate the switch to raise the gate. This is because the scripted sequence requires Princess to be on the other end, and she may not run with MacCready to perform this function. Short of using the console to activate the gate/toggle clipping, the solutions are to: use the other entrance, reload from a previous save, or attempt to move Princess in range of the event with the console command prid 000185d5 followed by moveto player. [verified]
pc Doing DLC quests before opening the path will end "Picking Up the Trail" resulting in MacCready's and Joseph's dialogue options to vanish and making entry into Vault 87 impossible. The only solution is to resort to a save that still has "Picking Up the Trail" as an active quest, and redo whatever progress has been lost or to use the console command ResetQuest 00014e8c and go to the Citadel to start the quest again. [verified]
MacCready may vanish from your settlement when you arrive there after selecting Paladin Danse as your companion on the Prydwen during the quest Shadow of Steel. [platforms tag needed][verification overdue]
pc Sometimes you may not be able to talk to MacCready or open up his inventory. He also fails to follow the player during fast travel. Entering the console command prid 0002a8a7 followed by moveto player will bring MacCready to the player, but does not correct the issue. [verified]
MacCready will become hostile and attack the player if the player attempts to pickpocket Fred Allen and fails in Goodneighbor. [platforms tag needed][verification overdue]