Harper, a Beatles fan, joined cellist Yo-Yo Ma on stage and sang a rendition of With a Little Help From My Friends.
NAC music director Pinchas Zukerman opened the concert, which was in support of the National Youth and Education Trust, with Mahler’s Blumine.
That was followed by young Canadian pianist Avan Yu playing Chopin. Then, to the surprise of the audience, Ma came out for an encore with Yu.
But instead of intermission after that performance, Harper shocked the audience by coming on stage and sitting at a grand piano. With Ma on cello, along with two guitarists and a drummer, they performed the classic Beatles hit, with Harper providing the lead vocals.
CBC's Evan Solomon, who attended the gala, said Harper's performance stunned and wowed the crowd, and received high praise even from members of opposition parties who were also there for the evening.
Harper's appearance contrasts with the controversial comments he made last year about such musical galas.
He angered some in the arts community over cuts to some arts projects and by suggesting many Canadians don't sympathize with artists seeking funds to support such galas.
"I think when ordinary working people come home, turn on the TV and see … a bunch of people … at a rich gala, all subsidized by taxpayers, claiming their subsidies aren't high enough when they know those subsidies have actually gone up, I'm not sure that's something that resonates with ordinary people," Harper said then.