"The Wolverine" actor, 50, was performing to a sold out audience at the Manchester Arena as part of his "The Man. The Music. The Show." tour on the same day as the veteran British star turned 80.
In a touching moment he filmed and posted on Instagram, Hugh turned to the crowd and said, "I'm sorry I can't make your party, Ian. But I think I might annoy about 50,000 people," before leading a rendition of Happy Birthday, blowing a kiss and adding, "Love you, man. Happy birthday!"
Ian has marked his own 80th year by touring Britain with his own one-man show raising money for regional U.K. theatres, a trek that has raised more than $1 million (£770,000) for theatre companies outside London.
Another one of his "X-Men" co-stars, Patrick Stewart, who has also starred alongside him on stage, most recently in Harold Pinter's "No Man's Land", also marked his birthday with a touching tribute to his great friend.
Patrick, 78, posted a picture of himself and his wife Sunny with Ian on their wedding day in 2013, when "The Lord of the Rings" star officiated.
"Happy 80th birthday wishes to @IanMcKellen! Sunny and I are grateful for all the love over the years, whether as a dear friend, minister at our wedding, or colleague on screen and stage," he wrote.
Last week a host of British stars of stage and screen, including, Patrick, Michael Sheen, Judi Dench, Helen Mirren, Anthony Hopkins, and Derek Jacobi also marked Ian's 80th birthday by sharing their memories of him with The Guardian newspaper.