RB Salzburg batted away two bids for their midfielder in January as the Austrian side wanted to see the season out with Aaronson in their ranks. The clubs agreed on a £25m deal that would come into play at the conclusion of the 2021/2022 season provided United were still a Premier League side.
Aged just 21 years old, the American arrives at Elland Road with significant international and Champions League experience and will strengthen United's midfield options ahead of the 2022/2023 Premier League campaign.
Here, we take a look at Aaronson's impressive career so far:
1. February 2020
Brenden Aaronson makes his full debut for the US Men's National Team in a 1-0 victory over Costa Rica.
Photo: John McCoy
2. November 2020
Aged just 20 years old, Brenden Aaronson is named in the Major Soccer League Best XI, a prestigious accolade which recognises the MLS' most elite performers, after starting all 23 games of the season for Philadelphia Union.
Photo: Emilee Chinn
3. December 2020
Brenden Aaronson scores his first senior international goal for the USMNT in a 6-0 win over El Salvador. He also bags his first assist in the same game.
Photo: Michael Reaves
4. November 2020
Brenden Aaronson helps Philadelphia Union to win the first title in its history - the Supporters' Shield, awarded to the MLS side with the best regular season record.
Photo: Mitchell Leff