Marshall was born in Los Angeles, California, the son of Barbara Wells, a nurse, and director/producer Garry Marshall; he is also the nephew of Penny Marshall. His two sisters, Lori and Kathleen, are also in entertainment. Scott had an interest in film since his childhood as his father recalled of his son's early efforts in film "I'd make him a little wooden airplane and he would take it immediately and burn it, and start to film it, flaming, crashing!” Also, “Later, we got a pool and he would get his friends to drink tomato juice and then he'd shoot at them and they would dive in the pool and the tomato juice would come out. It ruined the pool."

Marshall studied film directing at the AFI Conservatory where he directed his short film Waving Not Drowning. It later screened at the AFI/Los Angeles Film Festival. He was also one of two directors of Sabrina, the Teenage Witch during the 2002-03 season. Marshall also played bass guitar in the 90s indie rock band Chavez.

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