As a teenager, Lennon played with the Real Salt Lake youth academy in Arizona. He signed with English team Liverpool in 2015, and joined their youth teams. Lennon returned to Real Salt Lake in early 2017, ahead of agreeing a season-long loan with the club. Initially expected to play a developmental role, injuries to key starters thrust Lennon into the starting lineup as the season progressed.[3]
On December 20, 2017, it was announced that Lennon would join Real Salt Lake on a permanent basis.[4] During the 2018 season, he made an appearance at rightback in an early defeat, eventually establishing himself as the club's starter at the position.[5]
On December 2, 2019, Atlanta United acquired Lennon from Real Salt Lake in exchange for $150,000 in General Allocation Money and $150,000 in Targeted Allocation Money.[6] He made his competitive debut for the club in early 2020, appearing in Atlanta's 3–0 victory over Motagua in the opening stages of the CONCACAF Champions Cup.[7] Lennon quickly established himself as a starter,[8] growing into a more advanced leadership role by the time he reached his third season with the club in 2022.[9] In November 2022, he signed a three-year contract extension with Atlanta.[10]
On January 8, 2018, Lennon received a call-up for the United States men's national soccer team for a friendly against Bosnia and Herzegovina.[12] Lennon also holds an Irish passport making him eligible to represent the Republic of Ireland. He is quoted as saying, "I love the bloody Eire" in an interview with Fox Sports.
On January 21, 2022, Lennon was called into his first World Cup Qualifying camp with the U.S. Men's Senior team.[13]
