Born in Los Angeles, California, Manella received a Bachelor of Arts degree from Wellesley College in 1972 and a Juris Doctor from the University of Southern California Law School in 1975.

She was a law clerk for Judge John Minor Wisdom of the United States Court of Appeals for the Fifth Circuit from 1975 to 1976.

She was legal counsel to the Subcommittee on the Constitution, United States Senate Judiciary Committee from 1976 to 1978.

She was an Assistant United States Attorney of the Central District of California from 1982 to 1990.

Manella first expressed an interest in serving as a judge on the United States Court of Appeals for the Ninth Circuit in 1995, when she wrote a June 7, 1995 letter to senior presidential advisor George Stephanopoulos expressing her interest in such a seat.