He was the manager of the Tagajō Senior High School basketball team before he made a name for himself. His voice-acting debut was the OVA Megazone 23 as the motorcycle-riding Shinji Nakagawa, while his television debut was the anime Bosco Adventure as the cowardly Otter. Yamadera's break-out role as a voice actor was the anime Ranma ½, in which he played the dual role of Hibiki Ryōga and the Jusenkyō Guide. In 1991, Yamadera joined forces with Toshihiko Seki and Noriko Hidaka to form the acting unit Banana Fritters (バナナフリッターズ, Banana Furittāsu), which disbanded in 1995. In October 1997, Yamadera became a host on the TV Tokyo children's variety show Oha Suta.

Yamadera married fellow voice Actress Mika Kanai in May 23, 1993. They divorced in 2006, although it wasn't publicized until 2007. He married Japanese voice Actress Rie Tanaka on June 17, 2012, with a ceremony held in Hawaii on January 3, 2013.

In 2011, Yamadera took over the role of Koichi Zenigata in Lupin III media from Gorō Naya.

As part of Adult Swim's April Fools' 2018 joke, he guest-voiced T.O.M. the animated host of Toonami, an American action cartoon block that frequently features Japanese anime. Starting at midnight, the show's bumps and typical anime programming switched to Japanese audio with English subtitles, with the disclaimers, text and even the [adult swim] logo all being shown in kana and kanji. In keeping with the joke, T.O.M. was voiced by Yamadera, introducing new shows in his native Japanese as well as doing a video game review of Nier: Automata. Ironically, and perhaps to further the prank, the normal English voice actor for T.O.M., Steven Blum also voices Spike Spiegel in the English dub of Cowboy Bebop, a role Yamadera shares with him in the original Japanese.