Milnes (1747-1837) was the eldest son of a successful Wakefield cotton manufacturer John Milnes and Mary Shore, the daughter of Edward Shore of Norton Hall. He entered the Royal Horse Guards in March, 1770, as a Coronet, becoming a lieutenant in November 1772 and finally captain in 1776.
In this full-length portrait, he stands in a landscape with his horse and groom beyond. It measures 4ft 2in x 3ft 4in (1.27 x 1.02m).
Rated as one of the finest Wright of Derby works left in private hands (it was last on the market in the 1950s), and in superb condition (the frame is possibly the original chosen by the artist), it was estimated at $1m-1.5m, but established a new auction record when it sold for $6.4m (£3.37m) plus buyer's premium to London dealer Jean-Luc Baroni.
The top price of the sale was St James the Greater, a late religious work by Rembrandt. Despite its status as a 'seen' picture (it was among the exhibits at last year's Maastricht), Sotheby's found an anonymous buyer towards the top estimate at $23m (£12.1m).
