Marshall was born in the New York City borough of the Bronx, the son of Marjorie Irene (née Ward), a tap dance teacher who ran a tap dance school, and the late Anthony Wallace Marshall (1906- 1999), a director of industrial film and later a producer. Comedy On Tap- Garry Marshall Interview He is the brother of actress/director Penny Marshall and Ronny Marshall Hallin, a TV producer. His father was of Italian descent, his family having come from Abruzzo, This Jewish boy's life will make you laugh (and get a bit verklempt?) and his mother was of English and Scottish descent; His father changed his last name from "Masciarelli" to "Marshall" before Garry was born. Marshall was baptized Presbyterian and also raised in the Lutheran religion for a time. Thunderstruck He attended De Witt Clinton High School and Northwestern University and is a member of the Alpha Tau Omega fraternity, where he wrote a sports column for The Daily Northwestern.

Marshall began his career as a joke writer for such comedians as Joey Bishop and Phil Foster, and then became a writer for the Tonight Show with Jack Paar. In 1961 he moved to Hollywood, where he teamed up with Jerry Belson as a writer for television. The pair worked on The Dick Van Dyke Show, The Joey Bishop Show, The Danny Thomas Show, and The Lucy Show. They then adapted Neil Simon's play The Odd Couple for television. On his own, Marshall created Happy Days, Laverne and Shirley (starring his sister Penny), and Mork & Mindy, which were produced by his associates Thomas L. Miller, Robert L. Boyett, and Edward K. Milkis. He was also a co-creator of the short-lived sitcom Makin' It, BBC - Comedy - Shows A-Z Index which was also produced by the three men.

In 1984, Marshall had a movie hit as the writer and director of The Flamingo Kid. A consummate producer, Marshall wore many hats during this period of his career: most of his hit TV shows were created and executive produced by him. His first producing assignment came with the series, "Hey, Landlord" in 1966. He stepped up to the very next year, producing "The Lucy Show." Then came an avalanche of successes in producing: "The Odd Couple," "Laverne and Shirley," "Blansky's Beauties," "Mork & Mindy," "Angie," and, of course, "Happy Days." Marshall also launched independent productions, via his theatre (The Falcon in Toluca Lake) and in association with productions launched with talent he was grooming and working with for years. One such project was entitled, "Four Stars," which was directed by actress Lynda Goodfriend (who portrayed Lori Beth in "Happy Days"), based on a teleplay she had read at the Lee Strasberg Center, written by John Schulte and K. Mahony. It starred Julie Paris, the daughter of "Happy Days" director, Jerry Paris and film veteran Bert Kramer. Marshall went on to focus on directing, with a series of hits, such as Beaches, Pretty Woman, The Princess Diaries and Valentine's Day.

Marshall is also an actor, appearing on the television series Murphy Brown and in such movies as Soapdish, Lost in America, Never Been Kissed, Orange County, and Keeping Up With The Steins, as well as Jumpin' Jack Flash and A League of Their Own, both directed by his sister, Penny Marshall. He was also a judge on FOX's filmmaking-competition reality TV series On the Lot, which aired in 2007, and guest-starred in The Simpsons episode Eight Misbehavin'. He also appeared in two episodes of Happy Days as a drummer.

Theater credits include Wrong Turn At Lungfish, which he wrote in collaboration with Lowell Ganz, The Roast withJerry Belson, Shelves, and A New Musical with Paul Williams, which had its premiere at The Papermill Playhouse in Millburn, New Jersey in the Fall of 2007.

Marshall has a star on the Hollywood Walk Of Fame. His son, Scott Marshall is also a director.