Working for Nintendo since 1990, Martinet started voicing Mario at video game trade shows in which attendees would walk up to a TV screen displaying a 3-D Mario head that moved around the screen and talked. This system was called "Mario in Real-Time" or "MIRT". Martinet's video game voice over debut was as Mario in the 1995 Windows release of Interplay's game Mario's FUNdamentals. However, most were first exposed to Mario's voice in the landmark 1996 game Super Mario 64. He has voiced Mario, Luigi, Wario, Waluigi, Toadsworth, Baby Mario, and Baby Luigi in most games where in these characters speak. His voice work appears in the English and Japanese-language versions of the games, albeit his work is always in English (or in Italian…).
Martinet also provided voiceover acting for the boxers and the announcer in the Super NES title Super Punch-Out!!, Vigoro in Sega's Dreamcast and Nintendo GameCube RPG, Skies of Arcadia, and is the signature voice of Leapfrog! "Ribbit Ribbit, Let's Play!"
In film and TV, Charles played Michael Douglas' father in The Game (1997) and the whacky Arnie the Dinosaur in Nine Months (1995) with Tom Arnold and Hugh Grant. He’s played hundreds of characters from policemen to news anchors to thugs in various TV shows and was the host in the pilot for the Classical Music Channel.