Marshall was born in Kansas, the sixth of eight children in a Catholic military family. During her childhood, she lived in several places in the South. Most of her family currently resides in Alabama.
As a child, Marshall and her siblings produced plays at their home. Marshall decided to become an actress in the seventh grade after winning a citywide monologue competition in Huntsville, Alabama. Playing the comedic role of the maid in her middle school's production of Junior Miss led her to focus on character acting. She acted throughout high school and college, graduating from Birmingham-Southern College with a degree in theater.
Marshall moved to Washington with the intent of attending graduate school at Catholic University or American University, but enrolled in classes at the Studio Theatre instead, working as a maid to support herself. During the 1980s, Marshall performed in nine Studio Theatre productions, including a 1983 role as Jo, a pregnant waif in Shelagh Delaney's A Taste of Honey, termed Marshall's "breakthrough performance" by friend and Studio Theatre Artistic Director Joy Zinoman. She also drew attention for her roles in Studio Theatre's Miss Margarida's Way and My Sister is in This House, for which she received her first Helen Hayes nomination.
Other productions in which she performed during that time included Briar Patch at Arena Stage; The Stick Wife at Horizons; The Vampires at Woolly Mammoth; and Baby with the Bathwater at Round House Theatre in 1989, for which she won the Helen Hayes Award for Outstanding Supporting Actress in a Resident Production. She joined the Woolly Mammoth acting company in 1989.
Marshall's recent productions include Dead Man's Cell Phone at Woolly Mammoth in 2007; Martha, Josie and the Chinese Elvis at Woolly Mammoth and The Prime of Miss Jean Brodie, featuring Marshall in the title role, at Studio Theatre in 2006; and Camille at Round House Theatre and The Clean House at Woolly Mammoth in 2005, for which she received her fourteenth Helen Hayes nomination.
Marshall began teaching acting at Round House Theater in 1985. Since 1988, she has taught at the Duke Ellington School of the Arts. She is also an adjunct drama instructor at Georgetown University. In her spare time, Marshall runs several miles a day and has dabbled in crafts such as drawing, metal work and pottery.
