Marshall spent house surgeon and registrar years at Wellington Hospital. Whilst Registrar to Verney Cable, he assisted at a number of the early cardiac catheterisations, and well recalls the delight shown by Verney during the second such investigation, when the catheter proceeded directly from the RV into the aorta in a patient with Tetralogy of Fallot.
Marshall proceeded to Hammersmith Hospital in 1953, where he initially enrolled as a PG student, and then for nine months in 1954 he was appointed SHO.
He then became Medical Registrar, Cardiothoracic Hospital, Sully in South Wales from 1954 - early 1956.
Marshall returned to Wellington Hospital as Medical Tutor (1956 - 1957) and as Staff Health Officer (1957 - 1958).
He worked in general practice in Khandallah 1958 - 1959. Marshall held an appointment in the Cardiology Department, Wellington Hospital, 1958 - 1991. He was appointed Visiting Physician, Wellington Hospital in 1960 and held this post until 1989.
In Cardiology, Marshall ran an out-patient clinic and also had an ECG reporting session once / week.