Ed McMahon related this story in his 1998 book, For Laughing Out Loud: My Life and Good Times:
It was my father who told me about my great-great-great-grandfather, Patrick Maurice Mac-Mahon, the president of France. General Patrick Mac-Mahon was an Irishman, but according to my father, Napoleon III loved him and was instrumental in his becoming the president of France in 1873. And according to my father, in his honor his favorite sauce was named Macmahonaise, which was eventually shortened to mayonnaise. People have sent me cookbooks that seem to confirm this derivation of mayonnaise. It's possible we really were related to the Irish president of France. But like many of my father's stories, it doesn't matter if every part is true. It's a story that belongs in the most prominent position on the top shelf.Does it matter if any part is true? According to Wikipedia, "mayonnaise made its English language debut in a cookbook of 1841." And the ancestors of Ed's supposed great-great-great-grandfather emigrated from Ireland to France in the mid 1700s, while the parents of Ed's paternal grandfather, Joseph F. McMahon, emigrated from Ireland to Massachusetts more than a century later. It's pretty clear that Ed McMahon did not descend from the president of France—nor even from the amiable sidekick who laughed at all of the president's jokes.
[Photo credit: "Ed McMahon" by Alan Light]