Jethro Tull spent his early years wandering from settlement to settlement with his uncle Johnny. His uncle was a grifter and a conman, and he was obsessed with building up enough of a nest egg to hit the New Vegas strip full force. One day while heading up north, Johnny decided to stop into Novac for the night. As they slept in their rooms, a half-starved, drugged out raider snuck into their room, slit Johnny's throat, and took everything they owned.
Jethro woke up and was horrified to see his murdered uncle in the bed across from him. He began screaming for what seemed like eternity until a woman came in and slapped him across his face. That woman was Jeannie Crawford, and she saved his life. She gave Jethro odd jobs to do around Novac and let him stay in his own room. Over the years, Jethro talked to all sorts of folks passing through, and he picked up a large number of skills. However, it was Doc Pilkinson that really changed Jethro's life. He stopped in one day and ended up staying in Novac for three years, tending to the wounded of the travelers there. During that whole time, Jethro was by his side, studying everything that Pilkinson would teach him.
Life in Novac was good and simple, but Jethro felt the call of adventure stir within him. He packed up his medical supplies and headed off to the nearest NCR recruiting station, and he was brought into their forces as a medic. It only took two years before Jethro was completely disillusioned by the soldiering life. He saw too much senseless death, and he couldn't stand to be a part of it any longer. He quietly quit the NCR and moved back to Novac to serve as their in-house doctor, with the help of his new, incompetent protege Ada Straus.