Jimmy Fallon and Justin Timberlake kicked off the show with a medley of music, catchphrases and characters.
Steve Martin, Taylor Swift, Tom Hanks, Paul McCartney, Melissa McCarthy and Keith Richards also joined the party.
McCartney performed an impromptu duet of I've Just Seen A Face with Paul Simon.
Other musical guests included Kanye West and Miley Cyrus.
"Tonight is like an enormous high school reunion," 15-time host Martin said as the show began.
'Saturday Night Live' has become a US TV institution and numerous cast members - including Dan Aykroyd, Chevy Chase, Eddie Murphy, Amy Poehler, Adam Sandler and Kristen Wiig - have gone on to carve out Hollywood careers.