Forward Semenyo turned and deftly back-heeled the ball past Chelsea goalkeeper Robert Sanchez from an Erling Haaland cross in the 72nd minuteto delight the City supporters.
“Everything happened so fast to be honest,” Ghana’s Semenyo, who joined City in January, told the BBC. “It came straight to me and I had to improvise myself as quickly as I can.”
“I have never competed for trophies like this before, so everything is new to me. Hopefully, we can finish the job off.”
The former Bournemouth player’s goal sparked what had been a cautious game into life with both sides having chances in the last 20 minutes, Rayan Cherki forcing a fine save from Sanchez and teammate Matheus Nunes hitting the post.
City captain Bernardo Silva, who is leaving at the end of the season, described the victory as very special.
“It’s really nice to finish this way and maybe we can still have a small dream that we can still fight for the Premier League,” the Portugal midfielder said.
City trail leaders Arsenal by two points in the title race going into the last two games of the season.
Semenyo said: “I don’t think there will be much celebration tonight – it is getting ready for Tuesday.”
City face sixth-placed Bournemouth in the league on Tuesday.
Silva received the FA Cup from Prince William and raised it aloft alongside defender John Stones, who is also leaving the club, to wild cheers from the sky-blue end of Wembley Stadium.
It was City’s eighth FA Cup triumph and followed victory in the League Cup in March, in what could also be Guardiola’s last season after 10 years in charge.
But it was a seventh successive domestic cup final defeat – the fourth in the FA Cup – for Chelsea, who had looked to the showpiece match for some relief after a turbulent season and two managerial changes.
Enzo Fernandez, Joao Pedro and Liam Delap all had chances to score for Chelsea but failed to find the target in what for long periods was an even, if unexciting, clash.
The London club’s hopes of European football next season now look very slim, while their American BlueCo owners search for a new manager to replace interim boss Calum McFarlane, who took over from the sacked Liam Rosenior.
Rosenior lasted less than four months following the new-year departure of Enzo Maresca, with Chelsea having plunged down the Premier League to ninth place. (Reuters)