After 120 minutes of goalless action, the game was decided from the spot with United sealing a 7-6 victory in the shoot-out.
Solly March lifted his effort well over the cross bar with Victor Lindelof stepping up to confidently slam home the winner.
David de Gea made a number of key saves to keep Brighton at bay before the shoot-out, responding in style after a harrowing night against Sevilla three days ago.
Erik ten Hag’s side will return to Wembley on 3 June to try and deny City a potential treble and claim their second piece of silverware of the season having already secured the Carabao Cup.
In a quiet first-half, Alexis Mac Allister tested de Gea early on with a free kick from the edge of the box, drawing a decent save from the Spaniard.
Perhaps still reeling from Thursday’s harrowing Europa League exit, United laboured early on but found their feet in the contest as the half wore on. Bruno Fernandes drilled his side’s best effort just wide of Robert Sanchez’s post with Christian Eriksen’s scuffed effort hacked away by the Brighton goalkeeper late in the half.
Brighton started the second in control, pinning United back with Julio Enciso denied another wonder strike with de Gea palming over his vicious effort from inside the box.
United soaked up the pressure up until the hour mark before responding, with Antony finding space to operate down the right but failing to truly trouble the Brighton goalkeeper after finding his way through on goal.
While a livelier affair in the second period, there was a lack of cutting edge and incisive final ball from both sides with Brighton missing the focal point of Evan Ferguson, ruled out through injury, and Anthony Martial failing to make an impression on the game.
Brighton failed to punish United who were caught on their heels on a corner kick as Danny Welbeck burst in at the back post unmarked only to head his effort over the bar.
With extra-time looming, de Gea was called into action again, this time getting down to his left to deny Solly March, pouncing to collect the loose ball with Pascal Gross lurking.
Jadon Sancho and Marcel Sabitzer, both introduced off the bench, almost combined to give United a breakthrough only for the Austrian to head his effort just wide.
Rashford was twice denied in extra time by Sanchez who himself produced a string of saves to keep the game goalless.
With neither side finding the net, it went down to penalties with United holding their nerve to set up the first ever Manchester derby in an FA Cup final.
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