Mount, who was a graduate of Chelsea's famed academy, spent 18 years at the London club.
Now he is about to join its Premier League rival United in a $69 million (£55m) deal and is set to sign a five-year contract.
"Given the speculation over the last six months this may not come as a surprise to you, but it doesn't make it any easier to tell that I've made the decision to leave Chelsea," he said in a video message posted on Instagram. "I know some of you won't be happy with my decision, but it is what's right for me at this moment in my career."
"I wanted to tell you how grateful I've been for all of your support over the last 18 years," Mount concluded.
The midfielder joined Chelsea when he was six years old and became a fan favorite after rising through the ranks to become a key player in the club's Champions League triumph in 2021.
He made 195 appearances and scored 33 goals. He went from being named the club's academy player of the year in 2017, to winning the first team player of the year award in 2021 and 2022.
During his time at Chelsea, the 24-year-old Mount also established himself as a regular for England.
Now he is set to begin the next chapter of his career at United, with manager Erik ten Hag making him a key target ahead of the club's return to the Champions League next season.
Mount's contract at United will have the option of a further 12 months.
United will also have to pay an extra £5 m ($6.3m) to Chelsea based on him meeting certain targets.