Simms was born in 2236. He grew up in Megaton and understands better than anyone what it takes to survive and thrive in the wasteland. Big and imposing yet warm and friendly, Lucas won his position by a unanimous vote of the people of Megaton (most of whom genuinely love the guy). He serves as mayor, sheriff, judge, and just about any other position that's needed. It's also no secret around Megaton that Lucas Simms is fulfilling some weird childhood cowboy fantasy. He named his son Harden after the Old West outlaw John Wesley Harden. Simms is also a member of a secret society known only as "the Regulators."
He often contests the title of Mayor with Colin Moriarty. While Moriarty claims Megaton to be 'his city,' every resident comments on how Simms is the real Mayor. Simms warns the player of talking to Moriarty, saying the same to his son. Likewise, Moriarty despises Simms.
Sometime after the Lone Wanderer's escape from Vault 101, the Lone Wanderer alerted Lucas to Burke and his plan to detonate Megaton's atomic bomb. After Lucas confronted Burke, he turned around and began to lead Burke out of Moriarty's Saloon. However, Burke shot Lucas in the back, killing him. 
If Lucas is killed, there are unique dialogue choices with Manya, Leo Stahl and Harden.
If the Lone Wanderer chooses to tell Lucas about Burke's plot, it is possible for them to save Simms by killing Burke before he gets to shoot Simms. This also spawns a dialogue choice of Lucas commenting on how he "must be getting slow", and he will thank the player character for saving his life.
If the Lone Wanderer chooses to tell him about their plan of detonating the bomb, he will turn hostile along with every other Megaton settler.
If the player character chooses not to save him and then kills Burke, the player character receives positive Karma and has the option of looting both of their bodies with no additional negative Karma. They can then receive the reward for disarming the bomb from Lucas's son, Harden.
xbox360 When Mister Burke kills Lucas, a floating blood splatter can sometimes be found walking the same path that he once did. It can cause the game to freeze and you will need to restart the console. [verified]
pcps3 Rarely, even though you kill Burke before he kills Lucas, the latter may still die. He will simply walk out of the saloon and collapse.[verified]
pc On rare occasions, Lucas Simms may follow you out of Megaton and stay behind you regardless of where you travel to. However, he is not hostile, although his health bar does show and he is wielding his gun.[verification overdue]
↑Afterward - Moira Brown: "Heck, if you get Simms drunk, he'll tell you that his dad died because of the Wanderer, even though he saved the town."
↑Afterward - Moira Brown: "Just shipped out another crate of survival guides, and the caravans just can't seem to get enough of them. Sheriff Simms says they've really put Megaton on the map—pretty ironic, since I've had to redraw that map about a million times since the first edition came out 20 years ago."