After acting for many years as apothecary to Bethlehem Hospital, London, and obtaining a practical knowledge of diseases of the brain, he was created a doctor of medicine by the University of Aberdeen, 17 September 1816. Haslam established himself as a physician in London. To comply with the regulations of the College of Physicians in London, he entered himself at Pembroke College, Cambridge, and kept some terms there, but took no degree.
He was admitted a licentiate of the College of Physicians, 12 April 1824.
Haslam was distinguished in private practice by his prudent treatment of the insane, while his scientific publications and his contributions to periodicals gave him a good reputation. In an 1809 edition of a work on insanity he included a detailed description of a case which, some argue, is one of the earliest and clearest recording of what would be called schizophrenia in the 20th century.
He died at 56 Lamb"s Conduit Street, London, 20 July 1844, aged 80.