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On January 18, 2018, McKenzie signed a homegrown player contract with Major League Soccer side Philadelphia Union. He would make 20 appearances his rookie season with the Union, mostly starting alongside fellow homegrown centerback, Auston Trusty. McKenzie's appearances and performance on the backline earned him a nomination for the 2018 MLS Rookie of the Year Award.

McKenzie received a call-up to the United States U-20 squad to compete in the2018 CONCACAF U-20 Championship. McKenzie scored 3 goals during the qualifying group stages and captained the team in a 7–0 victory against Suriname.

On December 20, 2018 McKenzie received his first Senior call up for a friendly against Panama. On February 4, 2020, McKenzie earned his first senior cap for the national team as a substitute in a friendly match against Costa Rica.

McKenzie has spent time in the US development ranks, being called into U-15 and U-16 camps. In 2017, McKenzie was called into the United States U-18s for the Slovakia Cup. He would make 7 appearances for the team in 2017. His performances would earn him 2016–17 Conference Best XI honors for the US Development Academy Eastern Conference U-18s.

McKenzie's performance in the academy ranks earned him call-ups to the Bethlehem Steel FC roster, the Union's USL team. He made his professional debut against New York Red Bulls II in June 2016. Over two seasons, McKenzie made 8 appearances for the Steel as an amateur player.

Growing up in Bear, Delaware, McKenzie began playing club soccer for the Delaware Rush and Wilmington Rangers before being brought into the Philadelphia Union Academy. In December 2015, McKenzie committed to play college soccer at Wake Forest University starting in 2017, joining his academy teammates Matthew Real and Joseph DeZart. McKenzie made his first start for the Deacons as a freshman against Clemson University. He would register 16 appearances and 5 starts for the Deacons.