Some 20 fire engines and 120 firefighters and officers have been called to the property on William Street.
The fire brigade was called at 3:55pm, and crews from Chelsea, Kensington, Hammersmith, Battersea and other surrounding fire stations are at the scene.
Control officers have taken around 40 calls to the fire, which they say is currently “producing a lot of smoke”.
Just days ago the property recently completed the most extensive restoration in its 115-year history.
Mandarin oriental on fire! pic.twitter.com/iVCtM176NR
— CryptoTruffle (@ivanferretti) June 6, 2018