John Tarn was born in Shipton, Oxford in 1793. He was educated at St George's Hospital, where he was entered as a six-months pupil to Thomas Keate in June, 1812, and in November of that year took out a further six-months ticket. 
Early Naval History
John Tarn was appointed to the position of Assistant-Surgeon Royal Navy on 11 May 1813. He was appointed to HMS Beever in 1817 
He was appointed Surgeon in the Royal Navy 8 September 1818 and was Acting Surgeon on Sybille in October 1819 He served as Surgeon on board HMS President, HMS Salisbury, and HMS Melville between 1815 and 1820.  He was appointed Surgeon to the Satellite in 1821. 
He was employed as Surgeon Superintendent on the Brunswick Emigrant Ship from Cork to Quebec in 1825.
Surveying Expedition 1826 - 1830
John Tarn was employed as surgeon on the Adventure on the Surveying voyages of the Adventure and Beagle between the years 1826 and 1830.
Mount Tarn, a small mountain located on the southernmost part of the Strait of Magellan, in Brunswick Peninsula, about 70 km south of Punta Arenas, Chile was named after John Tarn who first ascended the mountain in February 1827 while traveling with Robert FitzRoy on HMS Adventure and later ascended it while traveling with Phillip Parker King in HMS Beagle, during their surveying voyage from 1826 to 1830. He participated in a hydrographic survey conducted in the area, through the collection and classification of flora and fauna species. Read a full account of the expedition in Proceedings of the first expedition, 1826-1830, under the command of Captain ... By Robert Fitzroy
Sir Gilbert Blane's Medal
John Tarn was awarded the Sir Gilbert Blane's Medal in 1841. and the Silver Naval Medal with Clasp (Sir Gilbert Blane's Gold Medal was a Fund in perpetuity, for the purpose of providing the means of conferring a gold medal, once in every two years, on such two Medical Officers, whether Surgeons, or Assistant-Surgeons acting as Surgeons, as shall produce the most approved journals of their practice, whilst Actually serving in King's Ships.)
John Tarn was employed as Surgeon Superintendent on five convict ship voyages to Australia:
In 1849, he was present at a dinner at the Australian Club in Sydney to farewell Captain King R.N., who was returning to Europe. In a later account of the event he was described as 'one of the surgeons who accompanied the Polar expedition' 
He retired with the rank of Staff Surgeon, and was Agent for Sick at Liverpool in 1849.
MRCS Aug 4th 1815; FRCS (by election) Aug 26th 1844
John Tarn remained unmarried.
In the 1861 Census he was recorded living at the house of his unmarried sister Mary at Newton Abbot, Devon. Mary was 71 years old. Also residing with Mary was their brother Miles a retired solicitor age 90, and unmarried sister Sophia age 69. They employed two house servants.
John Tarn died on 7 November 1877 at Knowles Hill, Highweek, Devon. 
 National Archives. Medical Journal of the Brunswick Emigrant Ship, by John Tarn, Surgeon and Superintendent, During which Time the Said Ship was Employed in Conveying Emigrants from Cork to Quebec. 5 April - 27 June 1825