Sie wuchs in den Ruinen von Washington D.C. auf, ohne zu wissen, was mit ihren Eltern geschehen ist. Im Alter von 10 Jahren traf sie einen Mann, der eine militärische Ausbildung genossen hatte und sie bei sich aufnimmt. Mit der Zeit wurde Reilly vom Mädchen zur Frau und lernte von ihrem Mentor alles, was es über den Kampf, Taktik und Überleben zu wissen gab. Als Reilly 22 war, starb ihr Mentor eines natürlichen Todes. Sie brachte seine Leiche zum Arlington Cemetery.
Anschließend wanderte sie durchs Ödland und übernahm einige Jobs. Mit der Zeit bekam sie einen guten Ruf. Im Alter von 26 gründete sie die Reilly's Rangers und übernahm die Aufgabe, die Ruinen von D.C. zu kartografieren und von Supermutanten zu befreien.
Reilly keeps notes of her teammates in her terminal which are written as reports. According to her computer files, she and Butcher are in a complicated relationship. In her terminal log about Donovan, she mentions having a contact in the Stählerne Bruderschaft.
If you constantly pick the meanest (bottom) speech option when first talking to her, Reilly gets mad and will never talk to you again.
Depending on how you wake her up, Reilly's legs are crippled and can be seen limping if you use your medical skill to wake her up. But she will look like normal if you pass Doctor Barrows' speech check to wake her up.
If asked why there are so many mutants in the area she tells you that Brick thought there is something in the area they are looking for, which is the F.E.V. virus which they need to change humans into mutants.
If you have the Gun Nut perk you get a speech option saying that you've heard of Reilly Rangers before, and that they pack serious firepower. This has no further effects.
There are a few inconsistencies between Reilly's dialogue and her notes. If you ask about Talon Company, Reilly will tell about an encounter with them in which Brick, the team's Heavy Weapons specialist, ALMOST fought with a detachment of them; where as in her report on Brick, Reilly states that Brick did indeed fire on (and nearly massacre) them. It is possible that these were separate encounters with the Talon Company, hence the differing outcomes. She also states that Theo, the ranger lost in the Statesman Hotel, was a full ranger, whereas his contract is marked in her notes as "probationary". It is possible however, that Theo was promoted after, or immediately before, his death and Reilly didn't yet write it in her log.
Do not anger Reilly at all in the occasion where you get the quest. If you then go and start the quest another way she and her rangers will become hostile. However the Underworld residents will not become hostile to the player and will attack Reilly.
Sometimes Reilly, when in Underworld's hospital, gets stuck in a dialogue with no exit options. This seems to occur when you listen to her quest suggestion Reillys Rangers, choose to think about it and return later, but the quest already appears on your Pip-Boy and states "Find Reilly in Underworld". Carrying out a quick save before talking to her is recommended.
If you have the Gauss Rifle, and you remain hidden, in Underworld's hospital, then shoot Reilly while she is still unconscious, she will fall off the bed and stand up, but you wont be able to talk to her, though you are able to revive her with the required 60 Medicine