Painting sells for double its valuation.
The colourful work, called Fete Champetre, sold for pounds 11,875, against its original estimate of pounds 6,000-8,000.
The painting, described as "a romantic vision of Spring", went to an anonymous buyer.
Mostyn was born in Liverpool and trained at the Manchester Academy of Fine Arts.
A spokeswoman for Christie's said the sale was "a very good result against its original estimate."
A total of 90 paintings and works on paper were included in the sale with the top lot being The Derby Day, by William Powell Frith, which achieved pounds 505,250