Marsha Garces Williams was born in Milwaukee, Wisconsin. She grew up in Shorewood, Wisconsin and attended Shorewood High School. Garces' father, Leon Garces, was a Filipino born in Ubay, Bohol who moved to the United States in 1929. He later served in the United States Navy during World War II. Her mother, Ina Rachel Mattila, was Finnish.

After working as the family's nanny, Garces worked with Williams as his personal assistant on films such as Good Morning, Vietnam (1987) and Dead Poets Society (1989). She was later the Producer for several of Williams' films. Together, they founded a film production company, Blue Wolf Productions, in 1991. Garces Williams has been involved philanthropically with organizations such as Doctors Without Borders and Seacology.

Garces married Robin Williams on April 30, 1989, following the 1988 divorce from his first wife, Valerie. When the pair were married, Garces was already pregnant with Williams' child. Garces met Williams in 1984 when she was employed as a nanny for Williams' son Zachary. Previously, Garces was a Painter working as a waitress. Their romantic relationship began in 1986. During their marriage, they had two children, Zelda Rae and Cody Alan, born in 1989 and 1991, respectively. In March 2008, she filed for divorce from Williams citing irreconcilable differences. Their divorce was finalized in 2010.