John Grant Stewart was promoted from hospital mate to assistant-surgeon in 1825  He was appointed Surgeon, Royal Navy in 1829.
He was employed as Surgeon Superintendent on three convict ship voyages to Australia:
Nautilus to Van Diemen's Land in 1838.
Egyptian to Van Diemen's Land in 1839.
British Sovereign to Van Diemen's Land in 1841
Fever Ship Eclair
In October 1845, The Standard reported of a fatal fever on board the steam sloop Eclair at Sheerness:
The Chichester, 50 gun frigate, has been appropriated by the Admiralty to receive the remainder of the patients suffering from the fever on board the Eclair. This morning the Chichester was towed by the African to Standgate Creek, and the sick removed from the ill-fated vessel.
Dr. John Grant Stewart (1829) on half pay, having volunteered to attend the sick, was put on board yesterday. The honour and humanity of the medical officers of the royal Navy, upwards of 20 have volunteered their services on board the unfortunate Eclair.
Deputy Inspector of Hospitals
In October 1845 following his heroic effort on the ill-fated Eclair he was promoted to the rank of Deputy Inspector of Hospitals. 
Deputy Inspector of Fleets and Hospitals West Indies 1847
In July 1847 he was appointed to the Vindictive, 59, flag ship, in the West Indies, as deputy inspector of fleets and hospitals on that station.
He married Ella, third daughter of William Fossett, esq., of the Admiralty at St. Peter's Walworth in August 1857.
John Grant Stewart can be found in the 1851 Census. He is 45 and resided in Lewisham, Kent with his wife Ellen age 35 and their infant son John age 2. John gives his birth place as Inverness, Scotland. Lydia Fossett age 24 is a visitor. The family employed two servants.
A daughter Jessie was born to John and Ellen Stewart at Greenwich Hospital on 8th September 1856. Jessie died at Greenwich Hospital aged 18 months on 31 March 1858.
Appointed Honorary Surgeon to Her Majesty the Queen
In 1859 - at the Comitia Majora held on Friday 30 September, John Grant Stewart, extra licentiate of the Royal College of Physicians, was admitted a member under the temporary bylaws. . In November 1859 it was announced that he was to be appointed Honorary Surgeon to her Majesty the Queen.
Presented to the Queen 1860
At a At a Levee at St. James Palace in 1860 he was presented to the Queen by the Duke of Somerset . In 1869 on the occasion of Her Majesty's birthday was appointed the Most Honourable Order of the Bath.
John Grant Stewart died in 1869 at Ivy Bridge, Devon.
His Obituary was published in the Evening Mail in December 1869:
The death is announced of Dr. John Grant Stewart, C.B., Late Director General of the Medical Department of the Navy. Dr. Stewart entered the navy as assistant surgeon, and obtained the rank of surgeon in 1840. He was specially promoted to be deputy inspector in 1845 for having volunteered hi services to go on board and take charge of the infected patients in the Eclair. In June 1861, he was made an Inspector General of Hospitals and Fleets, and for a short period was Director General of the Medical Department of the Navy from which department he had to retired through ill health. He was granted a good service pension in April last.
We regret to learn the death, on the 17th inst. at Ivy Bridge, Devon, of Dr. John Grant Stewart, C.B. Inspector General of Hospitals and Fleets. Our readers will remember the hard treatment Dr. Stewart received in his forced retirement from the Plymouth Naval Hospital, and we regret that he has survived so short a time to enjoy the good service pension given him as a placebo.
Dr. Stewart had seen much service, and his intrepid and self sacrificing conduct in connexion with the Eclair, fever ship will not be readily forgotten. He became Inspector General in 1861.
By the deaths of Drs. Bryson and Stewart, an honorary physicianship and an honorary surgeoncy to the Queen become vacant, in addition to the honorary surgeoncy vacated some months back by the death of Dr. Folds, R.N. The Director-General will have no difficulty in selecting names for submission to her Majesty as worthy of honour as those we have mentioned ; and those highly prized honours should not be allowed to remain long undistributed.