Mary Ann McSweeney was born in Aptos, California. She started playing piano at the age of five and then began studying the violin at age 8. In high school she fell in love with the acoustic bass while watching Ray Brown perform at the Concord Jazz Festival. She was chosen to play at the Monterey Jazz Festival at age 16 with the All Star band fronted by Thad Jones and Mel Lewis.
McSweeney has played with jazz legends Dizzy Gillespie, Joanne Brackeen and Lee Konitz, Gil Goldstein and Bucky Pizzarelli, and played for famous conductors, Leonard Bernstein, Lalo Schifrin, and John Williams. She has performed with Renee Rosness, Vic Juris, Ken Peplowski, Dennis Mackrel, Lewis Nash and Larry Goldings. Mary Ann performed with the Maiden Voyage and Diva Big Bands and performed with her quintet at the Sexial Jazz Festival in Portugal and the Mary Lou Williams Jazz Festival at the Kennedy Center.