Twitter went into a frenzy with many convinced that his striking look of large shades, grey hair and an impressive moustache was a cover-up.
He was compared to former US president Donald Trump while others quipped it was in fact Meghan in costume.
In a tweet @NickdeSemlyen wrote: “I refuse to believe that this isn’t one of the Beastie Boys.”
Another (@WayneTechSPFX) said: “Mysterious man at the coronation solved. It’s Trump.”
But others were quick to jump on the “insane” theories spiralling online.
One user didn’t understand how a 79-year-old man could be compared to a 41-year-old woman and the Royal Academy of Music tweeted to say it was “very proud to dispel” rumours.