The 24-year-old knows what it means to represent United and, as supporters have seen in his signing interview with Helen Evans, he is buzzing to get started as a Red.
Using his official Instagram account on Thursday evening, the midfielder made a promise to United fans that he will give absolutely everything for the shirt.
“I know the history and understand the responsibility that comes with this shirt,” writes Mason on social media.
“The most important for me is to gain your trust and I will work hard to do that.
“I won’t let you down Reds!”