He is known for playing tough or grouchy men in dubbed anime and video games. His career includes Hotohori in Fushigi Yūgi and Jin in Samurai Champloo. He also played numerous cameos and minor characters in Rurouni Kenshin, in addition to the major character of Hajime Saito. He also does Don Patch in Bobobo-bo Bo-bobo as well as Gabumon and other voices in Digimon. He is even the English voice of Jet Link, as well as Kisame Hoshigaki in Naruto and its sequel, Naruto: Hurricane Chronicles.
According to Anime News Network, Thornton has the most credits of any male voice actor who does English language dubbing and in such work he is second only in total credits to Wendee Lee. He has been in nearly two hundred voice-over productions for Japanese animation.
Thornton is also well-known for having played many different minor voice roles throughout the Power Rangers franchise (and similar series such as VR Troopers and Masked Rider).
Thornton has also had experience with ADR direction. Currently, he is co-directing the ADR for the popular anime series Bleach alongside Wendee Lee.
Thornton's career is difficult to track, due to the multiple names under which he has been credited.