Lopez's professional career began in 1984 when he appeared as younger brother Tomás in the short-lived ABC comedy series a.k.a. Pablo. In that same year, he was cast as a drummer and dancer on Kids Incorporated. Mario is a capable drummer and has been seen in his many television roles playing the instrument. In 1989, Lopez was cast as A.C. Slater in the popular television sitcom Saved by the Bell, a role which lasted five years. Lopez's character was a masculine, athletic student, helping to garner the actor a sizeable teenage-girl fan base. He maintains a close friendship with fellow castmate Mark-Paul Gosselaar.

Since his stint on Saved by the Bell, Lopez has enjoyed a career as host of a variety of television contest shows. He hosted Pet Star on the Animal Planet channel and was co-host of ESPN2's ESPN Hollywood. He is the captain of the L.A. Armed Response team on GSN's Extreme Dodgeball. Like fellow Saved by the Bell actor Dustin Diamond, he was featured as a celebrity guest on The Weakest Link.

On USA Network's "Pacific Blue," he played police officer Bobby Cruz. Lopez also served as the host of the syndicated series "Name Your Adventure". He also starred as the Olympic diving champion Greg Louganis in "Breaking the Surface: The Greg Louganis Story."

He was also one of the co-hosts of the syndicated talk show "The Other Half", which aired from 2001-2003. This show was a male version of the female ensemble talk show "The View". The Other Half featured 4 male co-hosts, including Danny Bonaduce and executive producer, Dick Clark.

As late as 2001, Lopez' likeness was featured in the Disneyland attraction Space Mountain in Anaheim. He played a space captain that informed guests of the adventure-to-come while they waited in queue for the ride.

In March 2006, he joined the cast of the daytime soap opera The Bold and the Beautiful playing the role of Dr. Christian Ramirez. On July 18, 2006, Lopez was released from his B&B contract. He guest-starred as a plastic surgeon who drives Christian Troy to jealousy when he sees him naked in the shower in an episode of FX Network's Nip/Tuck during the show's fourth season, which began in Fall 2006.

In the fall of 2006, Lopez appeared as a celebrity contestant for the third season of ABC's Dancing With the Stars, partnered with professional dancer Karina Smirnoff, who made her series debut. The series lasted ten weeks with Lopez and Smirnoff losing in the final round to former Dallas Cowboy running back Emmitt Smith and partner Cheryl Burke. Lopez's strong start in the program left some viewers wondering whether he had received dance training prior to joining the series. Initially, he said that he had not received any formal training, but he later revealed that, though he hasn't had formal ballroom training, he has studied tap and hip hop styles.

He was announced as a celebrity guest host for CBS's Extra on January 12, 2006. His first assignment was covering the Golden Globes.

Lopez color commented for the EliteXC mixed martial arts event alongside pro-wrestler Bill Goldberg.

Lopez is both the host with Carmen Palumbo and co-executive producer of Si TV's Dating Factory

On March 12, 2007, Lopez will be auditioning in a not-for-air episode of The Price is Right to take over for the retiring Bob Barker