The Canadian singer topped the bill at Barclaycard Presents British Summer Time in Hyde Park, and his four-year-old son Noah, who has beat the disease, was there to watch his dad perform.
The 42-year-old star told the sell-out crowd that he'd been worried about returning to the stage - after joking about the torrential downpour that had soaked the audience.
"This is exactly how it was in my mind. I shit you not," he said.
He added: "It's been two years since I've been on stage, and like any human of course I worry that whatever I had at one point might have gone.
"But after two songs, I'm even better than before."
He also paid tribute to his fans for their love and support.
He said: "There are no words for how much love, affection, gratitude, that I have in my heart on behalf of myself, my family, for your love, for your prayers, for your support.
"I want to thank you not only for tonight, but for every night, for everything you've done for me. Each one of you has made such a difference in my life."
And the crooner is evidently a bit of a reality TV fan, as not only did he name-check 'Love Island', but he had former 'TOWIE' star Megan McKenna supporting him.
"This isn't a concert, this is Love Island.... and just like Love Island, incredibly sophisticated people are going to come together to connect, to meet.... and maybe do it in a swimming pool," he joked.
Michael and his actress wife Luisana Lopilato cancelled all work commitments after Noah was diagnosed with liver cancer in 2016.
The singer previously spoke of the "hell" he and his family had been through since Noah's diagnosis.
Talking to the Herald Sun, the 'Feeling Good' singer said: "You know what? Hell seems like a really nice place to vacation compared to where we've been.
"I've been to hell. I don't talk about the whole story, not even to my friends because it hurts too much
"It's my boy. He's a superhero, he doesn't need to relive it over and again. But I've been to hell."
Michael and his wife are currently expecting a baby girl. The couple already have Noah, and another son, Elias, who was born in January, 2016.