The Italian, who celebrated his 26th birthday with a trip to Wembley on Sunday, was beaten twice within as many minutes by stunning second half strikes from Yaya Toure and Samir Nasri as City came from behind to eventually run out 3-1 winners.
Mannone, the hero of the Black Cats' penalty-shoot-out victory over Manchester United in the semi-finals, was powerless to resist as Toure and Nasri produced exquisite finishes to give Manuel Pellegrini's side the impetus to lift the season's first major trophy.
"I thought the first one was incredible, the way it dipped down," he said. "I didn't expect a shot like that, to be honest. It kept going away from me while I was diving.
"The second one as well was such a brilliant finish. It was the only spot it could go in.
"Unfortunately, we need to accept that they have got quality. But we need to be proud because the team did really well and we need to be at the same standard in every game from now on because we know we can do it."
Unfancied Sunderland enjoyed a sensational start to the game when lone striker Fabio Borini fired them into a 10th-minute lead, and, with Gus Poyet's game-plan frustrating City in attack and leaving them vulnerable in defence, the Wearside club reached half-time with their unlikely dreams not only intact, but enhanced.
However, Toure's 55th-minute strike, which came courtesy of an effortless, but devastating swish of his right foot from 30 yards, left Mannone grasping at thin air. And, when Nasri drilled a brutal half-volleyed attempt across the keeper and inside the far post less than two minutes later, the outcome seemed inevitable.
Substitute Jesus Navas' late third perhaps flattered City, but, as the men in blue celebrated on the final whistle, there was huge pride, as well as tears, among the red and white ranks.
Mannone said: "We are proud even if we are disappointed, like any Sunderland fan. We gave everything - at least we have no regrets.
"We went into the game, we surprised them. We played really well in the first half and in the second half, they only came back with two class finishes, to be honest.
"We were really in shape and we were playing our game, and I can't fault anyone of my team-mates because we were spot on with our plan.
"But unfortunately, we faced a great Manchester City side. They have got talents like Yaya Toure and Nasri, and I think everybody saw that."
Sunderland were backed by a travelling support of around 31,000 which made so much noise that Mannone revealed he and his team-mates were at times unable to hear each other.
He said: "It was unbelievable. I have never heard anything like that in a game. It was the most noise I have ever heard.
"I couldn't even talk to my defence, to be honest, at some points. The pressure, as well, we put on them... our fans were pushing us from behind, and that was great."
Poyet's men are back in cup action at the weekend when they head for Hull in the FA Cup quarter-finals knowing victory over the Tigers will earn them another trip to Wembley.
In the longer run, it is the business of securing their Barclays Premier League status which will be the main focus, and Mannone knows they must maintain the standards they set in north London if they are to complete their mission.
He said: "We have got a few more games in the league and we need to believe we will bounce back straight away.
"At the end of the day, if we win next Sunday, we are here again, so that's the greatness of the season.
"We need to keep believing and keep doing well, as we did on Sunday even when we were 3-1 down. We need to take pride from this and go on into the league."