Twenty fire engines and 120 firefighters took nearly six hours to bring the blaze under control by 10pm in the evening.
London Fire Brigade said a total of 36 guests and 250 hotel staff were evacuated, with no injuries reported.
Firefighters will remain at the scene throughout today, and fire investigators will then take over to establish how the blaze started.
The hotel – which is home to two-Michelin-starred Dinner by Heston Blumenthal and Daniel Boulud’s Bar Boulud - completed the “most extensive” multi-million pound renovation in its 115-year history last month.
The Mandarin Oriental said that it was too early to assess the full extent of the damage.
The hotel said in a statement: “Yesterday’s unfortunate fire at Mandarin Oriental Hyde Park, London has been extinguished. The fire did not extend to the Residences at One Hyde Park.
“To the best of our knowledge, all Mandarin Oriental guests have been accounted for and we are contacting guests who were not in the hotel at the time. All colleagues are safe and accounted for. Hotel guests are being accommodated at other hotels in the vicinity. We would like to express our appreciation to everyone who has conveyed their concern about this sad incident.
“It is too early at the present stage to assess the full extent of the damage. A full investigation is underway and will take time to complete.
“Further updates will be issued as appropriate.”