25 May 1858 Appointed Staff Assistant Surgeon.
2 July 1858 Promoted Assistant Surgeon 2nd/24th (2nd Warwickshire) Regiment of Foot. The vacancy on the Staff was filled by Gentleman James George Stewart Mathison.
1 Sept 1869 Exchanged with Staff Assistant Surgeon Eugene Valentine MacSwiney.
1869–1870 On duty at Fort Rowner Gosport.
14 Jan 1871 Assistant Surgeon 52nd (Oxfordshire Light Infantry) Regiment.
Malta 23 Feb 1871 Arrived from England on appointment. Joined 52nd Regiment.
Malta 1872 On garrison duty.
1 Apr 1873 Promoted Surgeon–Major. Moved to the Medical Staff Malta.
Malta 2 May 1873 Left for Gibraltar.
1873–1879 On duty in Gibraltar.
June 1879 Left Gibraltar.
Sept 1879 On duty at Galway.
1880–1881 On duty at Dublin.
1882 Medical Officer Military Prison Dublin.
Malta 10 Aug 1883 Arrived from England.
Malta 16 July 1884 Promoted Brigade–Surgeon.
Malta 16 June 1885 To England on leave.
1885 Married Eliza McDowell Orr a concert pianist, daughter of Frederick McDowell Orr and Mary nee Phelan.
10 Sept 1885 Returned to Malta.
Malta 1886 On garrison duty.
1886 Birth of a daughter Mary Margarita (Marita).
Malta 18 May 1887 To England on leave.
18 Aug 1887 Returned to Malta.
14 Nov 1887 Member of a committee appointed by the Governor to enquire into how the cholera of August 1887 was introduced into the islands.
Malta 9 May 1888 Brigade Surgeon J Colahan's
Report of the President of the Cholera Committee re the water supply to the villages.
10 Aug 1888 Resident with his wife at the Great Britain Hotel Valletta. Moved from Malta to Chatham, England.
Nov 1888 Change of station from Chatham to Dublin.
16 Oct 1889 Promoted Deputy Surgeon–General ranking as Colonel, vice Robert Wyatt Meadows.
1889 Birth of a daughter, Beatrice in Dublin.
Nov 1889 Change of station from Dublin to the Cape of Good Hope.
14 Apr 1894 Aged 58 years, was promoted Surgeon-Major-General. Appointed Principal Medical Officer in Ireland where he succeeded Surgeon Major General William Collis. Collis had only served in the post for two years but had to vacate his tenure as he had reached the compulsory retirement age of 60 years. Medical officers who voluntarily retired were not eligible for reinstatement but were liable to be recalled for duty up to 55 years.
1894 Birth of a son Frederick John in South Africa.
30 Mar 1896 Placed on the retired pay having reached the compulsory retirement age of 60 years. Surgeon Colonel Alexander Francis Preston succeeded to the vacant rank of Surgeon Major General via J Colahan. Moved to Australia.
1900 Death of his wife Eliza in Australia aged 37 years leaving six children under 13 years.