City's 1-0 win over Inter Milan in Istanbul saw Pep Guardiola's men become just the second side to win the Premier League, FA Cup and Champions League in the same season.
"This is what you work your whole life for. I'm so happy," an emotional Grealish told BT Sport as he broke down in tears.
"I was awful but I don't care. To win the treble with this group of players and staff is so special.
"Anyone that knows me know how much of a family person I am and how much I love football and this is what I've worked (for) my whole life."
Signed for a then Premier League record £100million in 2021, Grealish struggled in his first season at City, but has been pivotal to an all-conquering campaign.
And Grealish paid tribute to Pep Guardiola (52), who lifted his third Champions League as a coach.
"He's a genius," added Grealish. "I want to say thank you (to him). You've made this happen for me. You've put so much faith in me, buying me for a lot of money.
"Even last year when I was playing crap he stayed there with me and this year he's given me that platform to perform, so I just want to say thank you to him."
Kyle Walker (32) was left out of the starting line-up by Guardiola but came on as a late substitute, watched in the Ataturk Olympic Stadium by his parents.
"I'm living the dream. My mum and dad are in the stand and from where I've come from in Sheffield it's not easy," he said.
"I remember when my mum didn't have a pound for the ice cream van so for her to be here experiencing this with my, God bless her, I'm just so grateful."
