This article is from the of the muppets FAQ, by Craig Silverstein with numerous contributions by others.
Steve Whitmire (whose other credits include Rizzo the Rat, Bean Bunny, and Wembley Fraggle -- he's been a major player since "The Muppet Show"). He has been performing Kermit since Jim Henson's death. He also performs Ernie. Brian Henson has never performed Kermit.
Whitmire's first Kermit performance was in the 1990 special "The Muppets Celebrate Jim Henson."