"Here's to 72 years of joy you've given the world and why we broke up, I'll never know," Lewis told Martin as he presented him with a birthday cake.
Lewis paid a surprise visit to Martin at the end of Martin's performance at Bally's Hotel and Casino on the Las Vegas Strip.
"You surprised me," said Martin, who was obviously overwhelmed by the moment. "I love you, and I mean it."
The two made entertainment history beginning in 1946, with singer Martin playing the straight man to the comic Lewis. They split in 1956 and have rarely appeared together since.