At the end of 1824 he enrolled as a student of surgery at St Bartholomew's Hospital under the famous surgeon and lecturer John Abernethy (1764–1831).
After discovering the difficulties the poor had in obtaining medical treatment, Marsden sought to establish a free hospital in London for which "poverty and sickness are the only passports".
[1] In 1828 he set up a small dispensary at 16 Greville Street, Hatton Garden, Holborn, which was named the London General Institution for the Gratuitous Cure of Malignant Diseases.
A few years later, Marsden turned his attention to people with cancer and, in 1851, set up another small establishment in Cannon Row, Westminster.
They had four children, three sons and a daughter, but only one of them, Alexander Edwin Marsden, a surgeon (1832–1902), survived into adulthood.