Ganz was born in New York City, the son of Jean (née Farber) and Irving Ganz, an arts supply executive. Both of his parents were first-generation Americans born in New York. His mother's family is of Polish Jewish origin from the Białystok area; his father's family is of Hungarian Jewish origin from Maramaros County, in what is today northern Romania.
Ganz grew up in Queens, New York. He briefly attended Queens College, City University of New York, where he and his friend Mark Rothman wrote several comedic skits and shows for school productions. After Rothman's father Abe—a chauffeur who sometimes drove for The Mike Douglas Show—was able to pass a spec script of theirs to Tony Randall, the two got a try-out writing gig on Randall's hit TV show The Odd Couple. However, the producers of the show would only pay for them to come to Los Angeles one-way. Ganz and Rothman dropped out of college and headed west to take the job. After being fired—causing them to briefly live in their car and contemplate driving back across the country to New York—and then re-hired by producer Garry Marshall, the two became regular writers on the show; and Ganz eventually became Head Writer.
That led to a career in Hollywood, writing for a string of television situation comedies. After writing for the short-lived sitcom Paul Sand in Friends and Lovers in 1974, Ganz moved on to writing for Happy Days and co-created two of its spin-off series, Laverne and Shirley and Joanie Loves Chachi.
Ganz and Rothman's TV writing partnership dissolved after studio executives broke up the pair circa 1981. Ganz met his new writing partner, Babaloo Mandel, at The Comedy Store, shortly thereafter; and they made the jump to writing for films.
In 1982, Ganz and Mandel teamed up with fellow Happy Days alumni Ron Howard—who wanted to start directing—and Henry Winkler—who wanted to move away from his image as Fonzie—to make their first film, the low-budget comedy Night Shift, which was also actor Michael Keaton's first film. Ganz's second film outing, Splash, launched the careers of Tom Hanks and Daryl Hannah and earned Ganz a nomination for the 1984 Academy Award for Best Writing.
Ganz and Mandel went on to write several other films, four directed by Howard and one by Penny Marshall, alumna of Ganz's previous projects Paul Sand in Friends and Lovers and Laverne and Shirley. Five of their films have starred Billy Crystal, two have starred Tom Hanks, and two have starred Michael Keaton.
Two of Ganz's films are about baseball; Ganz is a passionate fan of the New York Mets. Ganz and Mandel's film Parenthood was semi-autobiographical and highly praised by critics and led to two different spin-off television shows.
Ganz and Mandel are also widely used as Hollywood script doctors, known for their reliability and fast turnaround time. Their screenwriting on several major films of the late 1990s and 2000s is uncredited, including Stuart Little and Stuart Little 2 as notable examples.
Ganz lives in Los Angeles with his wife of more than 30 years, Jeanne Russo Ganz. They have three children—Scott, Allie, and Simon—all working in entertainment. He is a member of the Wilshire Boulevard Temple.Night Shift (1982)
Splash (with Bruce Jay Friedman) (1984) (Academy Award Nomination for Best Original Screenplay (1984))
Spies Like Us (with Dan Aykroyd and Dave Thomas) (1985)
Gung Ho (1986)
The Money Pit (1986) (uncredited)
Vibes (1988) (with Deborah Blum)
City Slickers (1991)
A League of Their Own (1992)
Mr. Saturday Night (with Billy Crystal) (1992)
City Slickers II: The Legend of Curly's Gold (with Billy Crystal) (1994)
Forget Paris (with Billy Crystal) (1995)
Multiplicity (with Chris Miller and Mary Hale) (1996)
Fathers' Day (1997)
Liar, Liar (1997) (uncredited)
Where The Heart Is (2000)
Fever Pitch (2005)
Robots (2005) (with David Lindsay-Abaire)
Tooth Fairy (2010) (with Joshua Sternin, Jeffrey Ventimilia, and Randi Mayem Singer)
Parental Guidance (2012) (with Billy Crystal, Lisa Addario, and Joe Syracuse) (uncredited)
The Odd Couple (1970–1975)
Happy Days (also supervising producer) (1974–1984)
Laverne & Shirley (also co-creator and producer) (1976–1983)
Busting Loose (also co-creator and co-executive producer) (1977)
The Ted Knight Show (also co-creator and co-executive producer) (1978)
The Rita Moreno Show (1978)
America 2100 (1979)
Angie (1979) (director only)
Joanie Loves Chachi (also co-creator and producer) (1982–1983)
The New Odd Couple (1982–1983)
Amazing Stories (1986)
Channel 99 (1988)
Knight & Daye (1989)
Hiller and Diller (also executive producer) (1997)
Gung Ho (ABC, 1986–87)
Parenthood (NBC 1990)
A League of Their Own (CBS 1993)
Parenthood (NBC 2010)
Splash (Stan, the Statue of Liberty Tour Guide)
Parenthood (Ballpark Vendor/Amalgam)
A League of Their Own (Radio Sportscaster)
Mr. Saturday Night (Stan the Writer)
Greedy (TV Director)
Robots (Voice of Mr. Gasket)
Wrong Turn at Lungfish (with Garry Marshall) – Off-Broadway show, originally starring George C. Scott and Tony Danza
Ganz and Mandel were featured in The Dialogue interview series. In this 90-minute interview with producer Mike DeLuca, Ganz and Mandel discusses their 40-year partnership as it evolved from television to feature films.