The 21-year-old made his first-team debut for the Red Devils in May 2021, and he has recently spent time on loan with Stockport County and Hibernian.
The defender made 45 appearances for Hibernian during the 2023-24 campaign before returning to Old Trafford, and he was involved in his side's preparations for the current season.
However, it has now been confirmed that the defender has made the move to Cardiff for an undisclosed fee, signing a four-year deal with the Welsh team until the summer of 2028.
Man United announce Fish has joined Cardiff
"Will Fish has completed a permanent move from Manchester United to Championship side Cardiff City," read a statement on Man United's official website.
"Fish, 21, became the 240th graduate from our Academy when he made his senior debut in 2020/2021, as a late substitute in the final game of the season away to Wolves, having joined the club as a youngster."
Speaking to Cardiff's official website, Fish said: "I'm delighted. It took a while to get it over the line, but now I'm here, I can't wait to get going and embrace it.
"It's a huge club. The direction that the club want to go in, you've seen a lot of good signings in this window. Being a part of that is really exciting and I'm happy to be here. Hopefully, I can do my best for the team.
"I've spoken to people that have played here in the past and they only said good things. I've heard amazing things about the fans. I spoke to David Marshall, who was here and the fans love him. That just shows that if you do well, they'll back you forever. I just can't wait to step out on the pitch.
Fish signs four-year contract with Championship outfit
"Marshy has played the game for a long time. I dropped him a text and the first thing he said was what an amazing football club it is. Marshy, when you speak to him, you know that what he tells you is right, so I have full belief that I'm making the right decision."
Meanwhile, Cardiff boss Erol Bulut said: "He is a young defender, who has been at Manchester United for many years, so has had a good education from them.
"For the last two years he was in Hibernian, so we watched him and we were interested in him in January too.
"We hope that he will progress with us in a good way. He is still 21 years old, just starting his career and he is good on the ball, good in duels, so he will add to our playing style.
"Of course, he has to push to be in the team. He is not quite ready to play yet, so he will be with us after the international break and hopefully he can add positive things to our game."
Cardiff will continue their 2024-25 Championship campaign with a clash against Welsh rivals Swansea City on Sunday afternoon.