Gosselaar first came to public notice as the star of the hit television comedy series Saved by the Bell, in which he played Zack Morris, and which aired on NBC from 1989 to 1993. His character was adapted from the short-lived Disney Channel sitcom Good Morning, Miss Bliss; although that show was cancelled after one season, NBC executives believed that a similar show, with Zack Morris as the series lead, had potential for success. He reprised the role in two TV movies and a less-successful spin-off, Saved by the Bell: The College Years. In 1994, he appeared as Zack Morris in a second spin-off series Saved by the Bell: The New Class in the episode “Goodbye, Bayside – Part 2”, along with Mario López as A.C. Slater and Lark Voorhies as Lisa Turtle.

In 1996, he appeared in the TV-film She Cried No as a college student who date-rapes his best friend’s sister at a fraternity party. In 1998, he starred in the feature film, Dead Man on Campus. Later that same year, Gosselaar played the central character in the TV drama Hyperion Bay, which lasted 17 episodes. In 2001, he starred in the movie The Princess and the Marine, with Marisol Nichols. He also starred in the short-lived WB series D.C. From 2001–05, he played Detective John Clark on ABC’s NYPD Blue. After the series ended, he joined the cast of ABC’s Commander in Chief, which lasted only one season. He appeared on the HBO series John from Cincinnati. He then gained the starring role of defense attorney Jerry Kellerman in the Steven Bochco-produced Raising the Bar, which debuted on September 1, 2008, on TNT, then it was canceled in November 2009 after two seasons.

In 1996, Gosselaar married former model Lisa Ann Russell. Together they have two children: son Michael Charles (born 2004) and daughter Ava Lorenn (born 2006). After 14 years of marriage, Gosselaar and Russell announced their separation in early June 2010. Gosselaar filed for divorce June 18, 2010, and it became final in May 2011.

Gosselaar is a sports car enthusiast, race car driver, track cyclist, dirt biker and pilot. In 2005, he competed in the Far West Championships for track cycling. He won the Category 4/5 Sprint Championship event at the Encino Velodrome. He is also a Brazilian Jiu Jitsu Blue Belt under Rigan Machado.

On June 8, 2009, Gosselaar appeared on Late Night with Jimmy Fallon in character as Zack Morris to promote his show Raising the Bar, indicating that the name Mark-Paul Gosselaar is his stage name and declaring, as Zack, that he would participate in a Saved by the Bell “Class” reunion that is being spearheaded by Fallon. He has since agreed to participate in the reunion being organized by Jimmy Fallon on Late Night with Jimmy Fallon along with four fellow castmembers to date. On February 4, 2015, Gosselaar reunited with Mario Lopez, Elizabeth Berkley, Dennis Haskins and Tiffani Thiessen on The Tonight Show Starring Jimmy Fallon where they appeared in a Saved by the Bell sketch with Fallon.

In October 2009, he made his off-Broadway stage debut in Theresa Rebeck’s play The Understudy with The Roundabout Theatre Company. The show extended its limited New York run until January 17, 2010.

Gosselaar began filming the TNT series Franklin & Bash on March 28, 2010. The series premiered on June 1, 2011. On November 11, 2014, it was announced that the series was canceled, after four seasons. The FOX series Pitch cast Gosselaar in the main role of Mike Lawson, star catcher and team captain, in February 2016, and premiered on September 22, 2016. On May 1, 2017, it was announced that the series was canceled, after one season. He appeared in 2 episodes of Dinner at Tiffani’s.

Gosselaar was engaged to advertising executive Catriona McGinn in August 2011. They married July 28, 2012, at the Sunstone winery in Santa Ynez, California. They have a son Dekker (born 2013), and a daughter Lachlyn (born 2015).

In 2019, Gosselaar began playing the lead role of Paul Jackson (Rainbow’s father) in ABC comedy series Mixed-ish (spin-off and prequel series of Black-ish). He took over the role from Anders Holm, who played the character in the pilot episode.