[6] In 2009, at the age of 22, McKenna retired from playing due to an ongoing hip injury which prevented him from making a senior first-team appearance for Tottenham.

[5] After retiring from playing, McKenna started his career in coaching while studying a sports science degree at Loughborough University.

[5] In August 2016, McKenna left his role at Tottenham to join the academy set-up at Manchester United, becoming the club's under-18s manager.

[5] Along with former United player Michael Carrick, McKenna was promoted to the first-team coaching staff and replaced Rui Faria as José Mourinho's assistant manager, ahead of the 2018–19 Premier League season.

[16] McKenna's arrival saw a quick upturn in Ipswich's form, winning seven of his first ten games in charge, keeping seven clean sheets in the process.

[21] Ipswich finished the season in second place in League One to gain automatic promotion to the EFL Championship, following a 19-game unbeaten run.

[22] Ipswich started the 2023–24 season well and a run of four wins and a draw saw McKenna pick up the EFL Championship Manager of the Month award for September.

[28][29] Despite being approached by fellow Premier League sides Chelsea and Brighton & Hove Albion,[30] McKenna signed a new four-year contract with Ipswich until 2028.

[34] McKenna picked up his first win in the Premier League in a 2–1 away victory on 10 November against Tottenham Hotspur, bringing Ipswich out of the relegation zone.

"[5] Jim Magilton, the Irish Football Association's elite performance director, agreed with Boonen's appraisal of McKenna commenting, "He is a meticulous planner yet every session is spontaneous.

[36] Since his childhood, McKenna has been a supporter of Manchester United, with reports claiming that his love for the club influenced his decision to leave Tottenham Hotspur for the Red Devils in 2016.