Nell was born in Los Angeles and was raised in Ashland, Oregon, where her father Bill Geisslinger is still an actor with the Oregon Shakespeare Festival. While she was growing up, her mother Annette O'Toole worked frequently, taking her children with her on location. In 1998, Annette, Nell and Annette's other daughter Anna moved to Brentwood, California. Annette married Michael McKean in 1999. Both Nell and Anna attended Crossroads School in Santa Monica, California. Nell graduated from the school in 2001. Then she attended Bennington College in Vermont, but only stayed for one term. In 2002, Nell joined the acting company at the Oregon Shaekspeare Festival as an intern. She occasionally does musical and variety type shows with her mother and stepfather, in which she accompanies them on piano, guitar and provides backup vocals.