Mount joined Chelsea when he was aged sixMason Mount has told Chelsea fans a move to Manchester United is "what's right for me at this moment in my career".
The England international addressed supporters in a social media video on Tuesday.
Mount has agreed a transfer to United on a five-year contract for a £55m fee, plus £5m in add-ons.
The midfielder joined Chelsea at the age of six, and he realises some Blues fans will not be pleased with his move.
"I've made the decision to leave Chelsea," the 24-year-old said.
"I feel you deserve more than a written statement so I wanted to tell you directly how grateful I have been for all of your support over the last 18 years.
"I know some of you won't be happy with my decision, but it's what's right for me at this moment in my career."
Mount also thanked Chelsea's academy, backroom staff, previous team-mates and the managers he worked under since making his senior debut - Frank Lampard, Thomas Tuchel and Graham Potter.
Chelsea rejected United's first three bids for Mount before an agreement was reached.
He will be Erik ten Hag's first signing of the summer as the Dutch boss looks to strengthen his side following United's qualification for next season's Champions League.
Mount, who has scored 33 goals in 195 appearances for Chelsea since making his debut in 2019, missed the end of the domestic season with injury as the Blues laboured to 12th place - their lowest Premier League finish for more than 25 years.
Mount won the Champions League in 2021 with Chelsea and started during the side's 1-0 win over Manchester City in the final. He also won the Fifa Club World Cup and Uefa Super Cup with the Blues.
He will be the latest Chelsea player to leave the club this summer, with the likes of N'Golo Kante, Kalidou Koulibaly, Kai Havertz, Ruben Loftus-Cheek and Mateo Kovacic having already departed.
