The first production Marathon 1 aircraft (registered G-ALUB) left Woodley on 14 January 1950 for a sales tour of Australia and New Zealand. The aircraft was painted in BEA markings as "Rob Roy" in September 1951 and was demonstrated to the airline at Heathrow. During acceptance tests for British European Airways it was decided that the Marathon was not suitable to replace the de Havilland Dragon Rapide and the order was reduced to seven aircraft, none of which were accepted by BEA. Six Marathons were delivered to the West African Airways Corporation in late 1952 for operation in and between the British colonies in that region. They were replaced in 1954 by de Havilland Herons. The last three production aircraft were given increased tankage and sold to Union of Burma Airways which operated them in the region for several years.
Most of the returned and unsold aircraft were then diverted for use by the Royal Air Force as navigation trainers with the designation Marathon T.11. After internal modifications, most of the 28 aircraft taken on charge from early 1953 were used by No. 2 Air Navigation School at RAF Thorney Island, Hants. A total of 16 aircraft were transferred to RAF Topcliffe, Yorks in June 1958 when No.1 Air Navigation School relocated there. By February 1969, only eight were airworthy. Apart from mechanical unreliability, the main problem was tail-heavy trim, an absolute ceiling of 9,500 feet, and a rate of climb of only 300 ft a minute. The navigational trainers were retired in April 1959 and most were quickly scrapped. A few Marathons were operated by other UK military users including the Royal Aircraft Establishment.
Derby Aviation H.P.R.1 Marathon at Manchester Airport in October 1956. Three Marathons were acquired in 1955 by Derby Aviation, based at Burnaston airport near Derby and predecessor of British Midland Airways. The aircraft were used on scheduled services within the UK and to the Channel Islands until their withdrawal in December 1960. One aircraft was delivered to Jordan in September 1954 for the personal use of King Hussein. The Mamba-powered Marathon testbed was later fitted with Alvis Leonides Major radial engines and used as a testbed for the projected Herald. Dan Air purchased one model in September 1958. The aircraft never saw service and was scrapped at Lasham in May 1959. No surviving airframes are known to exist but the upper fuselage section of Marathon M.60 G-AMGW was stored at Woodley, United Kingdom as part of the Miles Collection