Richardson is well-known for his various roles, including the Joker in The Batman, for which he received two Daytime Emmy Award nominations.

His other notable roles include F Is for Family, Barney Rubble in The Flintstones: On the Rocks, the Mauler Twins and Monster Girl in Invincible, and Chairman Drek in the video game Ratchet & Clank.

Richardson's other credits include Captain Gantu in Lilo & Stitch and its franchise, Shnitzel in the Chowder pilot (and was later replaced by John DiMaggio), Goro in Mortal Kombat, the second voice of Skulker on Danny Phantom, Sarevok in the Baldur's Gate series, Jolee Bindo in Star Wars: Knights of the Old Republic, Crunch Bandicoot in the Crash Bandicoot games, Sai Sahan in Elder Scrolls Online MMORPG series, Tartarus from Halo 2, Chairman Drek in Ratchet & Clank, Antauri in Super Robot Monkey Team Hyperforce Go!, Openly Gator from Queer Duck, Doctor Payne and others on The Proud Family, Dark Laser on The Fairly OddParents, Maurice from The Penguins of Madagascar (replacing Cedric the Entertainer), Slam Tasmanian and Tech E. Coyote on Loonatics Unleashed, and the voice of Exile in the 1990s animated series Road Rovers.

He was the voice of Heihachi in the PlayStation 2 game Soul Calibur II, though he is listed in the role as Victor Stone.

In 2005, he voiced Trigon on the animated series Teen Titans, taking over the role from Keith Szarabajka.

In 2006, he appeared in the comedy Clerks II in which he plays a cop who notices the words "Porch Monkey 4 Life" written on the back of Randal Graves' work jacket; he also voiced the elder dragon Terrador from The Legend of Spyro game series.

His voice roles in 2011 included Panthro in the Thundercats series, Martian Manhunter on Young Justice and Bulkhead, one of the Autobots in Transformers: Prime.

Later, he provided the voice of Judge Michaels in Tyler Perry's first animated film, Madea's Tough Love.