July 31, 1984
Mario and Luigi are featured in the crowd in the arcade game Punch-Out!!.
They reappear in the arcade Super Punch-Out!! since that game reuses the same background.
April 1, 1987
In the NES version of Punch-Out, Mario is the referee of the matches.
He was left in Mike Tyson's Punch-Out!! and Punch-Out!! Featuring Mr. Dream as they are all basically the same game.
May 18, 2009
While the final game doesn't include any direct reference to the Mario series, one of the producers of the game, Kensue Tanabe, revealed that Mario was considered as the referee, like in the NES version, but was ultimately scrapped because they thought he wouldn't fit with the new style of the game.
Additionally, Princess Peach was briefly considered as a secret opponent, but was scrapped due to concerns about violence towards women. The final game instead includes Donkey Kong as a secret opponent.
Super Mario Maker
September 10, 2015
One of the unlockable costumes that Mario can wear in the game by using a Mystery Mushroom is based on Little Mac, and it's unlocked in the 100 Mario Challenge mode or alternatively by using the Little Mac amiibo from the Super Smash Bros. series. It's based on Mac's appearance from Super Smash Bros. for Nintendo 3DS and for Wii U (in turn based on Punch-Out!! for Wii), and when worn it replaces the sound effects with sounds from the NES Punch-Out!!.
Copyrights for both series are held by Nintendo.