Bethlehem Steel on March 2, 2019. Aaronson appeared as an academy player playing for
Bethlehem Steel FC during their 2017 season after coming through the
Philadelphia Union academy. On September 30, 2017, Aaronson made his first start for Steel FC in an away draw versus
Tampa Bay Rowdies. Aaronson made 21 appearances for Steel FC and scored his first goal for the professional side against
Atlanta United 2.
Philadelphia Union On September 17, 2018, it was announced that Aaronson would join the
Philadelphia Union at the beginning of their 2019 season. On March 17, 2019, the homegrown player scored his first professional goal in his MLS debut, helping Philadelphia tie against
Atlanta United. Considered to be a back-up playmaker for the Union, injuries and suspensions gave Aaronson the opportunity to start and excel in the first team as both
playmaking number 10 and left-sided box-to-box midfielder. At the end of his rookie season, Aaronson finished second for the 2019
MLS Rookie of the Year Award, with three goals and two assists in over 1,640 minutes played. Aaronson was the youngest player out of the five finalists. The
2020 season became a breakout year for Aaronson who finished the season with 31 appearances across all competitions and scoring 4 goals. Aaronson's performance during the season earned him several accolades include being named to the 2020
MLS Best XI for the regular season and the
MLS is Back Tournament. The Union finished the season with the best league record earning the team's first trophy, the
2020 Supporters' Shield.
Red Bull Salzburg in 2021. After persistent rumors, it was announced on October 16, 2020, that Aaronson would join
Red Bull Salzburg effective January 2021 after the MLS season ended. While the transfer fee was undisclosed, Philadelphia announced that it would be the highest transfer fee paid for an American
homegrown player from MLS. Early news reports indicated the fee is US$6 million up front, with US$3 million in possible incentives. Aaronson made his debut for Salzburg on January 25, as a substitute during a 2–0 victory over
Rheindorf Altach. in 2021. Aaronson scored his first goal for Salzburg on February 10, 2021, scoring the game-winning goal during an eventual 3–1 win over
Austria Wien. On May 1, he won his first piece of silverware with Salzburg as the club defeated
LASK in the
Austrian Cup final by 3–0.
Sky Sports reported the transfer fee to be £24.7 million. He made his senior league debut for Leeds as part of the starting eleven in their season opener on August 6, with a 2–1 home win over
Wolverhampton Wanderers. On August 21, Aaronson scored his first
Premier League goal after forcing an error from Chelsea goalkeeper
Édouard Mendy in a 3–0 home win over
rivals Chelsea, helping Leeds secure their first victory over the Blues for 20 years.
Loan to Union Berlin On July 9, 2023, following Leeds'
relegation from the Premier League, Aaronson joined
Bundesliga side
Union Berlin on
loan for the 2023–24 season.
Return to Leeds 's match against
Fulham's match on 13 September 2025. Aaronson returned to Elland Road for the
2024–25 season, putting in powerful performances in his five games in August, which featured him scoring twice in two league ties, including the added-time equaliser in the 10 August 3-3 season home opener against Portsmouth. His two goals plus his assists earned him the fans
JD Sports Player of the Month Award for August 2024. Aaronson finished the
2024–25 season with a career-high nine goals, adding two assists. On April 21, 2025, Aaronson was promoted back to the
Premier League with Leeds following a 6–0 win over
Stoke City at
Elland Road and
Sheffield United's 2–1 loss to
Burnley at
Turf Moor. On May 3, Aaronson won the EFL Championship title with Leeds following a 2–1 win over
Plymouth Argyle at
Home Park. On January 7, 2026, Aaronson scored his first brace with Leeds United in a 4-3 loss to
Newcastle at
St James' Park. == International career ==