Most of the venue will be taken over for two weekends in August and one in September for the marathons.
The venue sits roughly halfway along the marathon route. It will hold registration and briefings for the majority of the 800-strong workforce for the men's, women's and Paralympic marathons.
All of The Mermaid's spaces will be used for the sporting events with the exception of the River Room suite which has been hired by sportswear brand Asics for events on 5 and 12 August to celebrate the Olympic marathons.
Alicia Duncan, general manager of The Mermaid, said: "We are delighted that we have been chosen and are looking forward to working with the team."
The Mermaid sits on the north bank of the Thames next to Blackfriars Station. It has a tierd auditorium holding up to 600 people and two function rooms with river views.
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