Marcy is first met taking refuge in the Museum of Freedom, Concord, in the company of Preston Garvey and what is left of his crew. Not much else is known about Marcy Long, other than the scant information given by her husband Jun.
At a point prior to the Sole Survivor's journey to Concord, Marcy, Jun and their young son, Kyle, were from the town of Quincy. Shortly after the Minutemen showed up to defend the town from the Gunners, Kyle followed the Minutemen around the town idolizing them.
Information found on the computer terminal in their former store in Quincy suggests that Marcy was upset that there were too few Minutemen showing up in Quincy. These fears came true when the Minuteman, Clint, betrayed the town and militia to the Gunners.
The town was attacked once again. Most of the Minutemen and citizens there were killed, including Kyle.
As a result of this tragedy Marcy is overcome with grief and comes across as very angry, depressed, and bitter, especially to the Sole Survivor. She does not seem to like contact from anyone and does not trust people easily.
If Mama Murphy dies during The Sight, Marcy Long will slip into even greater anguish, and blame the Sole Survivor, and Preston for not stopping them. Marcy even demonstrates a slight amount of character development at her loss. When the Sole Survivor first meet the gang at the Museum of Freedom, Marcy scoffs at Mama Murphy's prediction of Sanctuary, and only agreed to come along once it was explained to her that nobody had a better idea. When Mama Murphy dies, however, Marcy asks "What are we gonna do without the sight?" This suggests that she has come to respect Mama Murphy's talent and accept that it is indeed reliable fortune-telling.
Marcy will act a little less cynical if the settlement's overall happiness is high.
She can sometimes be seen having a conversation with Mama Murphy, in which she will act much less agitated than usual.
Upon reaching Sanctuary, Marcy will automatically assign herself as a farmer should there be any form of vegetable plants there, otherwise she'll just wander about until the Sole Survivor assigns her as such.