Barsi was the daughter of Hungarian immigrants József Barsi and Maria Barsi (née Benkő). Her parents fled the 1956 Soviet occupation of Hungary and then immigrated to Los Angeles. Maria Barsi wanted to be an actress herself, but later hoped that her daughter would be one, instead. Judith was discovered at a skating rink when she was five years old, but could still pass for age three. She went on to appear in over 70 commercials and soon appeared in movies. Later in her life, Barsi had growth hormone injections to help her growth.