Johnson is considered the most successful RAF pilot of the Second World War, bringing down at least 34 enemy warcraft.
He achieved the record despite missing the Battle of Britain due to a broken collarbone, and his reputation for crediting younger pilots with a victory to boost their confidence means that the true tally is probably higher still.
MacConnal-Mason sold the oil to a private UK buyer in the lead-up to its debut at the LAPADA Art & Antiques Fair in Berkeley Square.
The buyer then placed it with the private military members club in Mayfair where it is currently on display.
The portrait was painted by Leonard Harry Wells in 1945 at Lubeck in north Germany. Wells was born in Nottingham and trained at the Nottingham School of Art before going on to study at the Royal Academy Schools.
As previously reported, the picture was sold at Cheffins in Cambridge in January (for £28,000), with Johnson’s diary sold for £16,000 in July.