Mardy-Jones was elected an M.P. (L.) for the Pontypridd division in a by-election in July 1922 when he defeated the Liberal T.A. Lewis. He continued to represent this constituency until 1931, making his home at 16 Llantwit Road, Pontypridd. He visited India in 1927. He resigned from parliament in Feb. 1931 following accusations of allowing his wife the use of his M.P.'s railway travel voucher. He stood as an independent Labour candidate at Pontypridd in the general election of Oct. 1931, but received only 1110 votes.

Mardy-Jones attended a number of study courses in India, the Middle East and South Africa between 1928 and 1946. He served as the Staffing Officer of the Ministry of Supply, 1942-44, and as the Education and Welfare Officer with the British forces in the Middle East from 1945 to 1946. He became a popular public lecturer on foreign affairs and specialised on India and the Middle East. He was elected a F.R.Econ.S., and was appointed official lecturer to the National Coal Board on the economics of the coal industry. He published several volumes on the work of local government and ways of reforming the rating system including Character, coal and corn — the roots of British power (1949) and India as a future world power (1952).

He m. in 1911 Margaret, dau. of John Moredecai, St. Hillary, Cowbridge, Glamorgan. They had two daughters. He and his wife agreed to separate in Sept. 1933. He d. 26 Aug. 1970 at Harold Wood Hospital, Essex, at 90 years of age.