Having made her senior Everton debut in the side's 6-0 victory over Aston Villa in October 2020, Grace went on to sign her first professional contract with the club in April 2021, after enjoying a breakout season.
The midfielder reached another career milestone just a few months later, in October, scoring her first senior Everton goal against Manchester City in the Continental Tyres League Cup.
Clinton shared her pride at making the move to United and taking the next step in her career.
She said: “I’m delighted to be joining Manchester United at a very exciting time for the club.