In 1895 he opened an office in Bedford with George Grocock and worked in the Arts and Crafts tradition designing cottages, schools, shops and restoring churches including the Trinity chapel of St Paul's Church, Bedford. He was made a fellow of the Royal Institute of British Architects in 1900 and by the following year was the diocesan surveyor for Ely. Mallows rented rooms adjoining those of the landscape architect Thomas Hayton Mawson, and occasionally worked with him.
Mallows's major commission was Tirley Garth in Cheshire, where, from 1907, he designed the house and the major part of the gardens.
He remodelled the house and gardens of Cannons in Stanmore, London, between 1905 and 1908. The gardens were considered by some to be the greatest to be designed in the Edwardian era.[3]
From 1914 until his death, Mallows designed the house and gardens at Craig-y-parc, Pentyrch, Glamorgan, for the colliery owner Thomas Evans.[4]
Photographs contributed by Mallows to the Conway Library are currently being digitised by the Courtauld Institute of Art, as part of the Courtauld Connects project.[5]