The Chile international equalised for the Red Devils in the 24th minute of the contest, bagging his eighth goal in eight appearances at Wembley.
The goal cancelled out a Dele Alli strike in the 11th minute that had given Spurs an early edge.
Sanchez first delivered at Wembley in 2013, while playing against England in an international friendly. He scored both goals in a 2-0 Chile victory over the Three Lions.
Then, after joining Arsenal in 2014-15, Sanchez proved instrumental in helping the club win two FA Cup titles with outstanding individual performances at Wembley.
His goal in the 101st minute in the 2017 semi-final against Manchester City sent the Gunners to the final and he then opened the scoring in that final, which ended in a 2-1 win for Arsenal over London rivals Chelsea.
And prior to that he scored both goals in the 2015 semi-final, which was a 2-1 win over Reading, before again netting in the final - a 4-0 win over Aston Villa.
It also continues a history of success in the FA Cup for Alexis. He now has nine goals in 17 appearances in the competition since his first appearance in January 2015. During that time, only team-mate Romelu Lukaku has managed more goals in the tournament.
8 - Alexis Sanchez has scored eight goals in his eight appearances at Wembley Stadium for club and country, including four in three #FACup semi-finals. Menace.— OptaJoe (@OptaJoe) April 21, 2018
Overall, it was Sanchez's third goal for the Red Devils since his move from Arsenal in January.
He'll get a chance to continue his Wembley magic as Ander Herrera's goal put United into the final where they will face either Chelsea or Southampton on May 19.