Lucas began working as an Editor in 1964, mostly as an assistant Editor on commercials, eventually working her way up to assistant Editor to renowned Editor Walter Murch.

She met George Lucas while he was attending film school at the University of Southern California, and they were married from 1969 to 1983. They adopted one daughter, Amanda Lucas, who was born in 1981.

In 1974, Lucas and Verna Fields were nominated for the Academy Award for Best Film Editing for their work on American Graffiti, which was directed by George Lucas. In 1975, she edited the Martin Scorsese film, Alice Doesn't Live Here Anymore and was supervising film Editor on Scorsese's 1976 film, Taxi Driver. She received a nomination for the BAFTA Award for Best Editing with Tom Rolf and Melvin Shapiro for her work on Taxi Driver.

Lucas won the 1977 Academy Award for Best Film Editing with Richard Chew and Paul Hirsch for her work editing Star Wars. She was a supervising Editor on Martin Scorsese's 1977 film, New York, New York. Her last editing credit was as one of the three editors of the third film in the original Star Wars trilogy Return of the Jedi, directed by Richard Marquand and released in 1983. Lucas' last film credit was as Producer of 1996's No Easy Way.

Marcia later married Tom Rodrigues, a stained glass Artist who worked as a production manager at Skywalker Ranch from 1980 to 1983. In 1985, the couple had a daughter, Amy Rodrigues. Lucas and Rodrigues divorced in 1993.