In a joint statement the two companies said Hix's involvement with the hotel, which kicked off this spring, had not met up to the high expectations that were held by both parties.
"Conceptually a match made in heaven, it was assumed by many that similar philosophies shared by the brands guaranteed success," a spokesperson said.
"The reality is that, although the brands bonded over a desire to bring a new and edgy dining concept to Belgravia, the business model for this project has not achieved expectations."
"This has resulted in the amicable departure of Hix from the Belgravia hotel – a considered decision, which now leaves both parties free to pursue new and exciting avenues."
Both the restaurant and bar are now expected to close on 30 November.
Speaking exclusively to BigHospitality for a video interview in May, Hix admitted the Belgravia project had been a significant challenge.
“It is in a slightly, not odd area, but an area that I have never operated in before and I think a lot of people were quite surprised I did a venture in Belgravia. All the food critics didn’t understand why I was in Belgravia and they all gave me a bit of a hard time about it so that was a big challenge for us."
"The hotel is a lot busier now," Hix added. "We are getting a lot of locals in the restaurant. It was pre-designed before we entered into it so that was one of the challenges for me."
In a departure from Hix's signature British cooking style, the 65-cover restaurant showcased a menu with an international focus featuring dishes such as grilled poussin with pomegranate molasses and tagliolini with langoustines and chilli.
The Belgravia venture also included the second Mark's Bar in Hix's portfolio of venues as well as a private dining area and a 'cigarden' or cigar smoking terrace.
New York-based Thompson Hotels, which was founded in 2001 and made its UK debut with Belgraves, will now take over the running of the hotel's restaurants and bars before announcing its long-term F&B plans in 'the coming weeks'.
"In the meantime, guests can be assured of a quality dining experience that is in keeping with the high standards set by the hotel," a spokesperson added.
Watch a video interview below with Mark Hix which was recorded in May at the opening of his Shoreditch restaurant The Tramshed: