"When I started hosting, marijuana was illegal and you could smoke cigarettes anywhere you wanted," Leno joked on Thursday's program, referring to how much times have changed since he started the job.
Leno took over the show when Johnny Carson retired and has since consistently led the late-night ratings. However, he has twice been pushed out of the host's chair to make room for younger comics.
Leno, 63, was briefly replaced by Conan O'Brien in 2009, then was reinstated several months later when the show's ratings dipped. O'Brien now has his own talk show on TBS. Last year, NBC announced "Late Night" emcee Jimmy Fallon would take over "The Tonight Show" after the Winter Olympics wrapped. The program is also being moved from California to New York.
"I don't need to be fired three times," Leno quipped Thursday. "I get the hint."
"I'm really excited for Jimmy Fallon. It's fun to be the old guy and see where the next generation takes this great institution," Leno said at the end of the show. "And in conclusion, I want to quote Johnny Carson, 'I bid you all a heartfelt good night.'"