It sold to a single bidder on the telephone after no other interest emerged on the night.
The anonymous buyer was also the third party guarantor of the picture - meaning they had agreed before the sale to bid to a certain minimum level, presumably £20m, in return for a financial return if they ended up outbid.
The Lock was being sold by Baroness Carmen Thyssen-Bornemisza, whose private collection hangs mostly in a state-built extension to the Thyssen-Bornemisza Museum in Madrid. In a controversial move, the painting, which carried an estimate of £20m-25m, was sold to "safeguard the future of the private collection and allow the loan of other paintings to the museum to be secured for the foreseeable future".
It had previously been acquired by Sotheby's advisory board member Baron Hans Thyssen-Bornemisza for £9.8m in November 1990 - then a world record price at auction for any British work of art (although just shy of the £10m-15m estimate).
The price of the picture this time round was equal to George Stubbs's Gimcrack on Newmarket Heath which sold in the same rooms in July last and it ranks as second only behind Modern Rome - Campo Vaccino by J.M.W. Turner, which sold for a record £26.5m Sotheby's July 2010, in terms of the all-time auction results for historic British pictures.
Five works by Francis Bacon however have made higher sums at auction, including the record Triptych that made $77m (£41.4m) at Sotheby's New York in May 2008 and featured three separate canvases.
The Lock itself was one of six paintings that make up the artist's celebrated series of large-scale paintings of life on the Stour, and is the last to remain in private hands. It shows a lock-keeper at the gates of Flatford Lock while a barge waits in the basin for the water-level to drop.
When exhibited at the Royal Academy, the picture sold on the opening day - a unique event in John Constable's career - for 150 guineas to the celebrated collector James Morrison (1789-1857). It was inherited by his grandson Colonel James Morrison of Basildon Park, in whose family it remained until it was sold at Sotheby's in 1990.
In response to the exhibition of The Lock at the Royal Academy in 1824, The Morning Post wrote: "Mr Constable contributes a landscape composition which for depth, sparkling light, freshness and vigorous effect, exceeds any of his works."