Morris's first acting role was as a child when he appeared in the 1966 Star Trek episode "Miri". Star Trek was, at the time, shot at the same studio (Desilu Productions) that produced Mission: Impossible, where his father was working. He made his feature film debut in Star Trek III: The Search for Spock in a small role and later guest starred on Babylon 5, Star Trek: Deep Space Nine, and Star Trek: Voyager.
In the mid-1980s, he portrayed law student (later attorney) Tyrone Jackson on the CBS daytime drama The Young and the Restless. During a major storyline, his character used heavy theatrical make-up to appear Caucasian, in order to go undercover to expose an organized crime organization. In the 1990s, Morris played a recurring character, the Johnnie Cochran-inspired defense attorney Jackie Chiles, on the comedy Seinfeld. One story line as Chiles depicts him suing a tobacco company with great delight which rang true for the actor as his name Phillip Morris is also that of a major U.S. based tobacco company. Morris also co-starred in the TV remake of Mission: Impossible as tech wizard Grant Collier (son of Barney Collier, who was played in the original series by Morris's real-life father Greg Morris). He also said in an interview that he grew up watching the original Mission: Impossible, with series' lead Peter Graves, whom Morris came to consider his acting mentor. Through his childhood, Morris knew Graves' real-life children. The friendship continued, until Peter Graves' death on March 14, 2010, which devastated Morris. He voiced the supporting role of Dr. Sweet in Disney's 2001 film Atlantis: The Lost Empire as well as its 2003 sequel, Atlantis: Milo's Return. He played one of Will Smith's college professors on the NBC show The Fresh Prince of Bel-Air and played Dr. Clay Spencer on the then-UPN television show Girlfriends.
In the January 25, 2007 episode of the CW television series Smallville, Morris portrayed the DC Comics superhero Martian Manhunter, which is a recurring but small character in the series. He reprised that role on the show's sixth-season finale on May 17, 2007, as well as the episodes "Bizarro" and "Cure" in the seventh season and the episodes "Odyssey", "Prey" and "Bulletproof" in the eighth season. He reprised this role in the ninth-season episodes "Absolute Justice", "Checkmate" and "Salvation".
As a voice actor, he portrayed the villains Imperiex on Legion of Superheroes, and as the Immortal Caveman Vandal Savage on Justice League and Justice League: Doom. He voiced W'Kabi in the animated series Black Panther. He appeared on one episode each of the series CSI: Miami and Seven Days. Though largely unnoticed, Morris also was the voice of Paul the Apostle in Zondervan's The Bible Experience. Morris also made a cameo appearance as Miles Dyson in photographs in the television series Terminator: The Sarah Connor Chronicles. He provided the voice of the character Doc Saturday in the animated show The Secret Saturdays. He also played a major supporting role in the PlayStation 2 game Ratchet: Deadlocked, as Merc, one of the combat bots that accompany the main character Ratchet. He also worked as a voice actor on The PJs; according to Morris, co-creator and star Eddie Murphy didn't want to show up on some days to record the voice of Thurgood Stubbs, so the producers hired Morris to record Thurgood's lines, where he worked in a separate booth with the other actors, allowing the producers to replace Morris' recordings with Murphy's voice in case Murphy decided he wanted to record his dialogue. He did several voices in the animated film Dead Space: Downfall as Hansen and Glenn. He played Delroy Jones on the TV One series Love That Girl!, Saint Walker on the Cartoon Network series Green Lantern: The Animated Series, and Ultra Richard on the Cartoon Hangover series SuperF*ckers. He also voices Plank in the Sofia the First episode "The Floating Palace", Green Arrow and Hawkman in the film Lego DC Comics Super Heroes: Justice League vs. Bizarro League and Vandal Savage in the film Lego DC Comics Super Heroes: Justice League: Cosmic Clash.
Justice League: Doom