25 Mar 1795 Regimental Mate.
11 Apr 1795 Hospital Mate.
June 1795–Aug 1803 Served on Major-General Craig's Expedition, (11 June–15 Sept 1795), to capture the Cape of Good Hope from the Dutch.
17 May 1796 Surgeon Garrison of Cape Town.
Between 1 Oct 1801 and 6 May 1802, Somerville accompanied an expedition to the Orange River to buy cattle from the Bechuana tribes, to relieve the shortage of food at the Cape. He recorded the cultures of the indigenous people, the flora, and fauna of Bechuanaland, and published these in a Narrative of Journeys to the Eastern Cape Frontier and to Lattakoe 1799–1802.
Aug 1803–Apr 1805 In England.
25 Mar 1805 Deputy Inspector of Hospitals.
Malta 18 July 1805 Arrived from England.
1 Nov 1805 Left for Naples.
2 Nov 1805–21 Jan 1806 At Naples.
22 Jan 1806–6 Apr 1806 At Sicily.
7 Apr 1806–28 Nov 1806 In England.
29 Nov 1806–16 Apr 1807 At Sicily.
Malta 17 Apr 1807 Arrived from Sicily to take charge of the Medical Department during the absence of Ralph Green on the Egyptian Expedition.
Malta 2 Sept 1807 Had a son, Thomas Somerville, baptised by the Reverend Francis Laing MA (Oxf).
Malta Oct 1807 Left Malta for England, where he remained until Dec 1807.
Malta 15 Nov 1807 Thomas Somerville son of William Somerville and Ann Rutherford died at Malta.
Dec 1807–Mar 1808 In Sicily and Malta.
Mar 1808–Aug 1808 In England.
Aug 1808–Aug 1811 In Quebec. On the Staff of the Governor Sir James Henry Craig.
28 Dec 1815 Principal Inspector.
25 Dec 1816 Retired to half-pay.
1817 Fellow of the Royal Society (FRS).
11 Nov 1819 Physician to the Royal Hospital Chelsea.