McMahon is best known for his famous "Heeeeeeer's Johnny!" introduction line from The Tonight Show. McMahon was Johnny Carson's sidekick for all 30 years that Carson hosted the late night talk show, cheerfully serving as the foil for the comic's humor.
McMahon also hosted the talent show Star Search for 12 years in the '80s and '90s, co-hosted TV's Bloopers and Practical Jokes with Dick Clark and served as the pitchman for Publishers Clearing House (American Family Publishers). [Hollywood Reporter]