The Scotland international joined the Red Devils at the age of five and worked his way through the club's youth ranks before going on to make a total of 255 first-team appearances across all competitions.
McTominay scored 29 goals for Man United, netting 10 times in his most productive season for the club in 2023-24, and won the FA Cup and EFL Cup under former boss Erik ten Hag.
The 28-year-old was not considered a first-team regular by Ten Hag, though, and instead of sticking around at Old Trafford in the hope of breaking through, he opted to embark on a fresh challenge in Italy with Antonio Conte's Napoli.
Man United received a reported £25m from the sale of McTominay, who penned a long-term contract until the summer of 2028.
McTominay explains decision behind Man United exit
McTominay, who has chipped in with four goals and three assists in 18 appearances for Napoli this term, has admitted that is was a "straightforward" decision to swap Old Trafford for Stadio Diego Armando Maradona.
"It was a big decision, but in some ways it was quite straightforward," McTominay told BBC Scotland. "I saw the passionate [Napoli] fans, I saw the coach, I saw the players and I saw an opportunity.
"I took it. I didn't look back. It didn't take me long to make the decision because I knew that was what I wanted and I'll never have any regrets in my life. As soon as I put my mind to something I want to do it, that's it. There's no holding me back."
McTominay, who has started each of Napoli's last 12 Serie A games since making his debut as a substitute, has opened up on his emotional farewell from Man United and has explained why a transfer to Naples was needed in order to further himself.
"On my last day, I had to go and see everyone and it was not easy to say goodbye. But that's football. That's the life," said the Scotsman. "I spoke to [Ten Hag]. He expressed his thoughts on the situation. I said, 'It's my career, I am here to push myself on'.
"In this situation, I have to look out for number one and go with my gut feeling of what I want to do in my career and not be held back by any other external factors.
"It was amicable. I love that football club to bits, they have been my life for 22 years, but I have moved on now."
McTominay: 'It was a mutual decision to leave Man United'
Ten Hag publicly expressed his desire to keep hold of McTominay, but the home-grown midfielder was moved on by Man United in order the help balance their books and comply with Financial Fair Play (FFP).
"It was a mutual decision," McTominay. "It was a case of - there's an opportunity there, do you want to go?
"Obviously I know from their side FFP is difficult. It's not easy for clubs to make a decision in regards to home-grown players, but that was never in my thought process.
"My thought process was coming here, seeing the fans, the coach and hopefully doing really good things."
McTominay, who plays alongside fellow Scotsman Billy Gilmour under Conte, has helped Napoli climb to second in the Serie A table, just two points behind leaders Atalanta BC.
Man United, meanwhile, are languishing in 13th place in the Premier League table under new head coach Ruben Amorim, six points behind the top four.
Written by
Oliver Thomas