However, the contract was then suspended earlier this month.
Both sides declined to comment when contacted by PRWeek.
Will Woodhams, who was leading on the account for Mission, was recently promoted from the role of business growth director to chief growth officer.
Meanwhile Naomi Curtis, Grosvenors' head of media relations who announced Mission's appointment, has left for the role of head of comms at Sybarite Architects.
It has now emerged that Four Rain, part of Four Communications Group, had been appointed by Grosvenor Britain and Ireland to tell the "success stories" of the firm's estates across Belgravia and Mayfair.
Four Rain "will be working with the Grosvenor team to implement a long-term programme of activity that will ensure the districts remain the most sought-after London destinations to live, work and visit", a statement from the agency said.
The Mayfair and Belgravia estates came into the Grosvenor family in 1677, when it was a mixture of swamp, pasture and orchards. The group also owns other London properites and developments across the UK.
Mission has around 170 staff across offices in London, New York and San Francisco, with British founder Nicola Stephenson now based in the US.