Lohman was born and raised in Palm Springs, California, the daughter of Diane (née Dunham), a bakery owner, and Gary Lohman, an architect. She has one younger brother, Robert (born 1982). She is a vegetarian and has two cats, Monk and Clint. Her family had no industry connections, but at age nine, she played Gretyl in The Sound of Music at the Palm Desert's McCallum Theater. Two years later, she won the Desert Theater League's award for "Most Outstanding Actress in a Musical" for the title role in Annie. By the age of 17, Lohman had appeared in 12 different major productions and had been a backing singer for the likes of Frank Sinatra, Bob Hope and the Desert Symphony.

As a senior, she was an awardee of National Foundation of the Advancement of the Arts and was offered the chance to attend the Tisch School of the Arts, but declined.