"It's just stuff that you dream of when you're a little kid," the grinning winger told the BBC, describing the moment as "unbelievable".
"I don't think the foreign guys know as much as us English; we know what it means to play in the final. I lost one when I was at Aston Villa and I was desperate to win one."
Ilkay Gundogan (32) scored both goals for City, making history with a beautiful long volley 12 seconds after kick-off at a sun-drenched Wembley Stadium - the quickest goal ever scored in the FA Cup final.
City's lightning-quick start was no surprise to the German midfielder.
"We know how... we want to play the kick-off and we aim for that, to go long for Erling (Haaland) and then to get the second ball and try to attack quick," he said.
"The ball just was placed amazingly for me and I just had to hit it. Obviously it was quite a good strike and it went in and it was amazing."
Treble-chasing City, who captured their third consecutive Premier League title two weeks ago and can add their first Champions League title with a victory over Inter Milan in Istanbul next Saturday, were dangerous because they had a mix of everything, said Grealish.
"Obviously everyone's confident but we have got the older lads with more experience, we have the younger lads and we have so much belief in ourselves and so much confidence and I think it showed out there today," he said.
"I'm sure we'll have a good night tonight and then straight back on it tomorrow and Monday for a massive game next week."
Bruno Fernandes (28) scored United's lone goal with a penalty after a controversial handball decision against Grealish.
"I genuinely don't know how it's a penalty... I'm not even looking at the ball," Grealish said. "I can understand if I'm trying to block a cross and I'm looking at it, but anyway Gundo saved me."
Saturday marked an historic first FA Cup final between the Manchester rivals.
The treble has been achieved by only one other English club - Manchester United in 1999 - and Erik ten Hag's team would have loved nothing better than to spoil City's dream.
Asked if he planned to party on Saturday night, Grealish said: "Of course I am! I don't win the FA Cup do I every week? So I'm going to enjoy it of course.
"I'm going to enjoy myself and then obviously tomorrow, you know, straight back into training and I just can't wait for next week now. I love this team so much."
