Samuel's great-grandfather, another Nathaniel, was youngest son of John Merriman (1618–1670), a captain in the army of Cromwell (Waylen, Hist.

His mother, who was his father's second wife, was Mary, eldest daughter of William Hawkes of Marlborough, and niece to Sir Michael Foster the judge.

After hearing the lectures of Matthew Baillie and William Cumberland Cruikshank at the Anatomical Theatre in Great Windmill Street he attended in 1795 the midwifery lectures of Dr. Thynne and the Westminster Lying-in Hospital, but his clinical knowledge of disease was principally obtained by seeing the numerous patients of his cousin William (1766–1800), son of the elder Samuel Merriman.

In 1808 he was appointed physician-accoucheur to the Westminster General Dispensary, having previously received the honorary degree of M.D.

from Marischal College, Aberdeen, for which he was specially examined in London by Dr. Vaughan (afterwards Sir Henry Halford).

On 17 August 1809 he was elected to the like office at the Middlesex Hospital, where in 1810 he commenced his annual course of lectures on midwifery, and continued them regularly till 1825.

In 1822, when his consultation practice as a physician for the diseases of women and children had largely increased, he removed to Brook Street, Grosvenor Square, and he subsequently purchased an estate at Rodborne Cheney, Wiltshire.

Merriman resigned his post at the Middlesex Hospital on 7 March 1826, but continued to take a warm interest in the institution, and was one of the treasurers from 1840 until 1845.

Merriman illustrated with anecdotes his copies of ‘A Picture of the College of Physicians’ and Wadd's ‘Nugæ Chirurgicæ.’ He had also a fine collection of portraits of medical men.

In 1794 he came to London to complete his medical education, and was admitted member of the Royal College of Surgeons and of the Society of Apothecaries.