The Somerset dealer shares the space, 6 Masons Yard, with two other dealers.
The first is Guy Peppiatt, who has been in the premises since 2007 and previously shared it with drawings specialist Stephen Ongpin.
The other is Modern British art specialist Harry Moore-Gwyn, who first occupied the third floor gallery before moving down to the shopfront in 2021.
The trio plans to use the gallery for shared as well as individual exhibitions.
The first major event will be a mixed show after the first series of London autumn fairs.
Mitton and Moore-Gwynn are both standing at the British Art Fair (see story on facing page).
In the meantime, British watercolours and drawings dealer Peppiatt is hosting a loan exhibition in his Mason’s Yard, London, gallery until September 27 featuring a recent bequest to Eton College.
The Richard Amis bequest comprises works by JMW Turner, JR Cozens, Peter de Wint and David Cox.