The 18-year-old was one of many star performers in United's thrilling 4-3 Emirates FA Cup quarter-final win over Liverpool on Sunday, but had initially been selected for England's Under-21s.
However, national coach Gareth Southgate has now asked the Academy graduate to join his 25-man group.
The midfielder reported to St George's Park on Tuesday, along with United team-mates Harry Maguire and Marcus Rashford.
Mainoo has represented England at Under-17, Under-18 and Under-19 levels, but now has the opportunity to push for a first senior international cap.
Both games will be played at Wembley Stadium, with Brazil's visit scheduled for Saturday 23 March (19:00 GMT) and Belgium to follow on Tuesday 26 March (19:45 GMT).