In March 2024, Mainoo was called up to the senior squad for the first time for England's friendlies against Brazil and Belgium, after previously being drafted for the under-21s.

He made his debut aged 18 on March 23, 2024, as a 75th-minute substitute for Conor Gallagher in a 1–0 defeat to Brazil at Wembley Stadium.

Mainoo made his first-ever England start against Belgium once again at Wembley three days later, in which he picked up a good reception following the game where he came off after 75 minutes for James Maddison, and was eventually awarded man of the match on his first start.

The 19-year-old Manchester-born midfielder, who is also eligible to play for Ghana through his parents, has impressed at club level and earned his first senior cap for England in a recent friendly. 

However, under FIFA rules, Mainoo can still switch allegiance to Ghana as long as he hasn't played more than three competitive games for England before turning 21.

This has sparked a potential tug-of-war between the two nations for the talented youngster. Ghana has expressed their interest in Mainoo, with a representative from their FA, Randy Abbey stating their desire to "persuade" him to represent the Black Stars.

The youngster is one of five uncapped players included in Southgate's expanded squad, which also features established names like Harry Kane and Jack Grealish.

Southgate has a selection headache due to injuries and fitness concerns within the squad. He acknowledged the unprecedented situation and the need to assess players' fitness before finalizing the 26-man squad for the Euros by June 7th.

This selection has also seen some high-profile omissions, with Marcus Rashford and Jordan Henderson left out. Southgate explained that Rashford's lack of form and the emergence of other players were the reasons behind his decision, while Henderson's recent injury hampered his chances.

England will play friendlies against Bosnia and Herzegovina and Iceland before the Euros kick off in Germany. With the final squad announcement looming, all eyes will be on whether Mainoo will commit to the Three Lions or choose to represent Ghana on the international stage.

Preliminary England squad for Euro 2024:

Goalkeepers: James Trafford, Dean Henderson, Aaron Ramsdale, Jordan Pickford

Defenders: Jarrad Branthwaite, Jarell Quansah, Luke Shaw, Marc Guehi, Ezri Konsa, Lewis Dunk, Joe Gomez, Kyle Walker, Harry Maguire, John Stones, Kieran Trippier

Midfielders: Adam Wharton, Kobbie Mainoo, Curtis Jones, Conor Gallagher, Declan Rice, Trent Alexander-Arnold

Forwards: Ivan Toney, Ollie Watkins, Anthony Gordon, Harry Kane, Phil Foden, Cole Palmer, Jude Bellingham, Bukayo Saka, James Maddison, Jarrod Bowen, Eberechi Eze, Jack Grealish

Defenders: Jarrad Branthwaite, Jarell Quansah, Luke Shaw, Marc Guehi, Ezri Konsa, Lewis Dunk, Joe Gomez, Kyle Walker, Harry Maguire, John Stones, Kieran Trippier

Midfielders: Adam Wharton, Kobbie Mainoo, Curtis Jones, Conor Gallagher, Declan Rice, Trent Alexander-Arnold

Forwards: Ivan Toney, Ollie Watkins, Anthony Gordon, Harry Kane, Phil Foden, Cole Palmer, Jude Bellingham, Bukayo Saka, James Maddison, Jarrod Bowen, Eberechi Eze, Jack Grealish