The Spanish international, recruited into the United team in place of Sergio Romero, who competed in the previous stages of the cup, suffered a nightmare night in the semi-finals.
His first error came in the 11th minute of injury time in the first half, allowing Oliver Giroud's shot to beat him at his near post to put Chelsea 1-0 ahead.
The worst was to follow almost immediately at the start of the second as the excellent Mason Mount burst forward and fired from afar, seeing his effort wiggle under Gea's body and into the goal.
De Gea made a superb save to deny Giroud his second goal before Marcos Alonso's cross passed by Harry Maguire, again beaten to his close post, in the 74th minute.
United, who entered the semifinals after a streak of 19 games unbeaten, scored a late comfort as Bruno Fernandes scored from the penalty spot after an awkward challenge from Callum Hudson-Odoi on Anthony Martial.
But they could have few complaints about the outcome, with Frank Lampard's Chelsea enjoying the best of the game from the start, Reece James forcing Gea to a first save before Alonso squandered a golden chance.
A horrific collision between Maguire and fellow defender Eric Bailly resulted in a big delay in the action, the Ivorian defender having finally injured his head.
The incident may have troubled United, but de Gea would have expected better with Giroud's opener, his 16th FA Cup goal in seven years of English football for former club Arsenal and Chelsea and now in his fifth final .
"Well, I told my boys in the locker room that I would like to win another one, it would be my fifth," Giroud told BBC Sport.
"It's another final, another great game against Arsenal, so it's special for me personally, but we put it aside and we're very happy with the team's performance today."
De Gea's calamitous mistake for Mount's second ended United, with manager Ole Gunnar Solksjaer turning to substitutes Paul Pogba and teenage revelation Mason Greenwood with little effect.
Instead, the third came as Maguire stretched to prevent Alonso's center from reaching Antonio Rudiger, the ball entering the net at Gea's knee speed.
Solksjaer was forced to defend his post as fire keeper after the match, not for the first time this season.
"David knows he should have saved the second goal but it is done now," he said.
"It is difficult for a goalkeeper to make amends, except for David making two or three fantastic saves afterwards."
Lampard is now looking forward to seeing his team offer him his first silverware as manager in the final at Wembley on August 1.
"I hope so because we want to win things, and at the start of the season we wanted to be up there for the top four.
"Many people were denouncing us, but quietly I was still hoping," he added.
Tottenham sinks Leicester
Earlier, Tottenham Hotspur struck Leicester City's aspirations in the Champions League hard as two goals from Harry Kane helped Jose Mourinho's men win 3-0 at home.
A goal from James Justin on a strike from Son Heung-min gave Tottenham a quick lead before English striker Kane struck twice before half time with great goals after skillful counterattacks.
There was no goal in the second half, leaving Leicester still fourth, but in terms of points with Manchester United, who had a match in hand before the teams met in the last match of the season .
Chelsea in third will also have a double reason to rejoice, the pressure being released after a shock defeat against Sheffield United which left doubts about the qualifications in the first four.
Tottenham, well improved with only one defeat since the restart, rose to sixth place in the challenge for a place in the Europa League.
At the other end of the table, Bournemouth's hopes for survival were compromised by a 2-0 home loss to Southampton, a neighbor on the south coast.
He left the Cherries face down, three points behind Watford in 17th place, which made the shocking decision to fire manager Nigel Pearson on Sunday, the news was confirmed in a club statement, with Hayden Mullins as interim coach for the last two games. .
