After employment in advertising and printing, he followed his interest in Victorian painting – sparked by reading William Gaunt's Aesthetic Adventure at university – and moved to work at Bonhams auction house, where he established the watercolour and drawings department.
The gallery specialized in Victorian art – paintings, watercolours, and drawings – which at the time was unfashionable and often difficult to sell.
He was also involved in the rediscovery and sale of many lost works, including Frederic Leighton's Flaming June (1895), and a painting of Saint Cecilia by John William Waterhouse that was found rolled up in France.
He published several books, including a work on Victorian Painters in 1969; a biography of the art dealer Gambart, Prince of the Victorian Art World in 1975; and two books on individual paintings by William Powell Frith and William Holman Hunt: The Prince of Wales' Wedding in 1977, and Holman Hunt and the Light of the World in 1984.
Maas died at his home, "Martins", in Amberley near Arundel in Sussex, of renal failure and arterial disease.