Mount has completed a £55million move to Manchester United this summer after a contract dispute at Chelsea.
Instead of entering the final year of his deal at Stamford Bridge, the Blues decided to cash in while United won the race for his signature amid interest from Liverpool and Arsenal.
As a Chelsea academy graduate, it has left a sour taste in many fans' mouths after seeing a player come up through the system join a Premier League rival.
And that was exacerbated by new footage that has emerged of Mount and his father Tony sitting in the dressing room at Old Trafford.
Mount Sr can be hear saying: "Mason… this dressing room man… home mate, home.
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"Who would have thought ey? When I brought you up here as a little boy…"
Mount replied: "I know. Special isn't it."
And it has led to a furious reaction from Chelsea fans on social media, including talkSPORT's very own Rory Jennings.
He wrote on Twitter: "Slippery little Judas, him. Certainly had me fooled."
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Alluding to Thibaut Courtois' controversial Chelsea exit, one fan wrote: "He's now officially in the Courtois books by the way."
A third said: "Mount's Chelsea legacy has been completely tarnished no matter what you say."
Another posted: "Mount is genuinely ruining any legacy that he had with us."
While this fan put: "Dont EVER EVER EVER tell me Mount was a Chelsea boy through and through again."
Mount is the first signing at United this summer after long talks with Chelsea that threatened to collapse.
The Blues had priced the England international at £70m but United only wanted to pay £50m max.
But a compromise was made over a deal worth £55m, that could rise to £60m.
There has been something of an exodus at Stamford Bridge where 10 players have already left and more are expected.
Mateo Kovacic and Kai Havertz have joined Manchester City and Liverpool respectively while N'Golo Kante, Ruben Loftus-Cheek, Kalidou Koulibaly, Edouard Mendy and Cesar Azpilicueta have all departed as well.