The 19-year-old has put pen-to-paper on a two-year deal at the Stadium of Light and will join up with the Black Cats' Under-21 squad on July 1.
It caps a memorable few years for the young striker, who joined Altrincham's youth team at the age of 17 and hit 15 goals in 35 starts this season.
The joy of sealing a move to the north-east is in sharp contrast to the disappointment he felt when he was released by boyhood club Manchester United at the age of 13.
That crushing blow ensures Watmore will not get too far ahead of himself - so much so that he will continue his Finance and Economics degree while at the Black Cats.
"I've always recognised how volatile the football world is," he told the Daily Mail. "One minute you're making it, the next you're gone. So I've always thought, no matter how successful I get, I was always going to stick with my degree and make sure that was the main thing.
"I want to prove myself and hopefully I can. I'm looking forward to doing so. My main focus is just to become a better footballer.
"I'm looking forward to it, definitely. But I'm moving to live on my own, and I'm going to be in a full-time job while studying. It'll be hectic."
Watmore also admits the speed of the move caught him by surprise, with Sunderland senior professional development coach Kevin Ball moving quickly once he had spotted the striker's talents.
"It all happened within a week. They put in a bid, then they negotiated," he added. "Four or five days later they said they agreed and then two days after that they invited me up for the medical. It all happened very quickly.
"I wouldn't have predicted this two months ago. It's all been so crazy for me, and it's come out of nowhere. All this season I've just been thinking about doing well for Altrincham. I assumed I'd be there next season, and then this offer came along."
Watmore is also excited to work with new Sunderland boss Paolo Di Canio, who has introduced a much stricter disciplinarian regime since taking charge of the club in March - something Watmore agrees with.
"I think whatever changes he does bring will be for the better of the club," he said. "I'm very happy with being disciplined and I think it's very important. Keeping the players in line is a good thing.
"The aim is to be working under him, so I want to get that opportunity and hopefully if I play well, I might get the chance."