The midfielder was due to join England's under-21 side but has since been drafted into the senior setup after another top performance in United's 4-3 win against Liverpool in the FA Cup quarter-finals.
Mainoo is a graduate of United's youth academy and has only played 20 times for his club, but has caught the eye of supporters across the country - clearly impressing Three Lions manager Gareth Southgate.
What makes his call-up all the more impressive is that he won't be replacing any injured players in the squad, being added instead to provide competition in the midfield area.
"It's an honour to receive my first senior call-up," Mainoo posted on Instagram.
Speaking at the Nations League draw in February, Southgate said: "He's doing brilliantly. I'm not certain he's necessarily going to be a defensive midfielder, as such.
"He's quite a progressive player but he's had a fabulous start to his career and it will be good to monitor him as we go forward."
Mainoo could debut when England face Brazil on March 23rd, or Belgium three days later on March 26th.
