The deal for the 157-bedroom hotel's freehold is inclusive of debt and was paid for in cash. Completion of the sale is expected by the end of the month.
The sale is approximately £4m over book value. The proceeds will be used to repay £61m of MWB's debt, with the majority of the balance available for free use within the group.
On completion of the sale of the Park Lane site, MWB will have raised more than £230m from the sale of recent hotel investments, which includes the Howard hotel in London.
Richard Balfour-Lynn, MWB chief executive, said: "The sale reflects the strength and depth of the investment market for high-quality hotels that are well managed by internationally recognised major brands."
MWB owned 70% of the property, with the remainder held by a subsidiary of its long-term partner, The Sincere Company Limited of Hong Kong.
The hotel is managed by Marriott Hotels under a 35-year agreement. MWB was advised by Jones Lang LaSalle Hotels.
By Emily Manson