Garry Kent Marshall was born in The Bronx, New York on November 13, 1934, the son of Anthony Wallace Marshall (1906–99), a Director of industrial films and later a Producer, and Marjorie Irene (née Ward; 1908-1983), a tap dance Teacher who ran a tap dance school. He was the brother of actress/director Penny Marshall and Ronny Marshall Hallin, a television Producer. His father was of Italian descent, his family having come from San Martino sulla Marrucina, Chieti, Abruzzo, and his mother was of German, English, and Scottish ancestry. His father changed his last name from Masciarelli to Marshall before Garry was born. Marshall was baptized Presbyterian and also raised Lutheran for a time.

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. Their first television series as creator-producers was Hey, Landlord, which lasted one season (1966–67). Then they 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 Makin' It, which the three men also produced.

Marshall wore many hats during this period of his career: Most of his hit television series were created and executive produced by him. His first producing assignment came with Hey, Landlord in 1966. He stepped up the very next year, producing The Lucy Show. Then came successes in producing The Odd Couple, Laverne and Shirley, Blansky's Beauties, Mork & Mindy, Angie, and Happy Days. He launched independent productions through his theater (The Falcon in Toluca Lake) and in association with productions launched with talent he was grooming and working with for years.

In the early 1980s, he met Héctor Elizondo while playing basketball and became great friends. Elizondo appeared in every film that Marshall directed, beginning with his first feature film Young Doctors in Love. Elizondo once noted that he is written into all of Marshall's contracts whether he wanted to do the film or not.

In the opening credits of Exit to Eden (their eighth film together), Elizondo is credited "As Usual ... Hector Elizondo". In 1984, Marshall had a film hit as the Writer and Director of The Flamingo Kid.

Marshall received the Valentine Davies Award (1995) and Laurel Award for TV Writing Achievement (2014) from the Writers Guild of America.

In 1996, Marshall was awarded the Women in Film Lucy Award in recognition of excellence and innovation in creative works that have enhanced the perception of women through the medium of television. He was inducted into the Television Hall of Fame for his contributions to the field of television in 1997.

His son Scott Marshall is a Director and his daughter Kathleen Marshall is an Actress and Producer. In 1997, he co-authored the memoir Wake Me When It's Funny with his daughter Lori Marshall.

His theater credits included Wrong Turn at Lungfish, which he wrote in collaboration with Lowell Ganz, The Roast with Jerry Belson, Shelves and Happy Days: A New Musical with Paul Williams, which had its premiere at the Falcon Theater in Burbank, California, February 24, 2006. He portrayed the role of "director" on Burbank's "Lights... camera... action!" float in the 2014 Rose Parade. In 2014, Marshall appeared in a guest star role in a February episode in season 11 of Two and a Half Men.

In 2012, he was inducted into the National Association of Broadcasters' Broadcasting Hall of Fame.

On the morning of July 19, 2016, Marshall died at a hospital in Burbank, California at the age of 81 due to complications of pneumonia after suffering a stroke.