He worked in both watercolours and oils, and also worked as a book illustrator - providing the illustrations for Sir Sidney Lee's The Imperial Shakespeare. He contributed to a mural in the Houses of Parliament in 1910 along with Byam Shaw, Ernest Board and Henry Arthur Payne.
As art fashion changed Cowper increasingly exhibited his portrait paintings but still continued to produce historical and literary works.
He was a member of the Royal Watercolour Society, of the Royal Academy and of the Society of Graphic Art.
His The Ugly Duckling was voted the favourite painting by visitors to the Cheltenham Art Gallery & Museum in 2005.
Molly, Duchess of Nona
St Agnes in Prison Receiving from
Heaven the 'Shining White Garment'
La Belle Dam Sans Merci
A portrait of Fraunces, Beatrice, James and
Synfye, children of James Christie, Esq
Venetian Ladies Listening to the Serenade