The Beginners star accepted his first ever Academy Award - the trophy for 'Best Supporting Actor' - from last year's 'Best Supporting Actress' winner Melissa Leo.
Bowing to the crowd, who gave him a long standing ovation, Plummer gestured to the Oscar statuette and said: "You're only two years older than me, darling. Where have you been all my life?
"I have a confession to make. When I first emerged from my mother's womb, I was already rehearsing my Academy Awards speech... mercifully for you, I've forgotten it, but I haven't forgotten who to thank - the Academy, of course, for this extraordinary honour, and my fellow nominees... I'm so proud to be in your company."
Plummer also thanked director Mike Mills and Beginners co-star Ewan McGregor, calling him a "superb artist, who I would happily share this award with if I had any decency - but I don't".
He also paid tribute to producers Olympic Films, studio Focus Features and his daughter Amanda, "who always makes me proud".
Plummer finally thanked his "long-suffering wife Elaine, who deserves the Nobel Peace Prize for coming to my rescue every day of my life. Thank you so much."
More from the Oscars 2012:
> Oscars 2012: 84th Academy Awards live blog
> Oscars: Academy Awards 2012: Red carpet pictures
> Oscars 2012: Octavia Spencer wins 'Best Supporting Actress' for 'The Help'
> Oscars should be won, not purchased, says Academy
> Halle Berry cancels Oscars appearance
> Oscars 2012: Academy Awards winners list in full
Watch the trailer for Beginners below: