7 June 1934 Granted a Short Service Commission Lieutenant RAMC on probation. From 7 June to 31 March 1935, he was seconded to the City Hospital, Aberdeen, under the provisions of Art. 213, Royal Warrant for Pay and Promotion, 1931.
Malta 16 Nov 1935 Arrived from England as a Regimental Medical Officer.
Malta 1 Apr 1936 Promoted Captain RAMC.
Malta 12 Dec 1936 On home leave.
Malta 4 Feb 1937 Returned from leave.
Malta 7 Jan 1938 On home leave.
Malta 28 Feb 1938 Returned from leave.
Malta 3 Mar 1939 On home leave.
Malta 21 May 1939 Returned from leave.
Malta 30 Jan 1940 Returned to England.
1940 Married Constance Annie May Cook.
1 Apr 1940 Converted his Short Service Commission to a Permanent Commission as Captain RAMC, with retained seniority (London Gazette 19 April 1940).
29 Dec 1941 Granted the rank of Acting Major RAMC.
13 July 1942 Granted the rank of Temporary Major RAMC.
1942–1943 Assistant Director Medical Services GHQ Home Forces.
28 Oct 1942 Acting Lieutenant Colonel RAMC.
28 Jan 1943 Granted the War Substantive rank of Major RAMC.
1943–1944 Assistant Director Medical Services (Ops) HQ 21 Army Group.
28 Oct 1943 Temporary Lieutenant Colonel RAMC.
1 June 1944 Captain (War Substantive Major) A MacLennan was promoted Major RAMC.
1944–1945 Commanding Officer 16 Light Field Ambulance North West Europe.
1945–1946 Commanding Officer British Military Hospital Jhansi India.
1946–1947 Assistant Director Medical Services HQ Malaya Command.
1947–1948 Assistant Director Medical Services (Ops) GHQ Far East Force.
21 Nov 1948 Promoted Lieutenant Colonel RAMC.
1948–1949 Assistant Director Medical Services HQ London District.
1949–1950 Assistant Director Medical Services HQ Eastern Command.
1950–1952 Commanding Officer 26 Field Ambulance Korea.
10 Apr 1952 Temporary Colonel L/RAMC.
1952–1953 Assistant Director Medical Services HQ 6 Armd Div British Army of The Rhine.
1953–1957 Assistant Director Medical Services HQ 3 Infantry Division.
1957–1961 Assistant Director General Army Medical Services War Office (AMD 1).
2 May 1958 Promoted Colonel L/RAMC.
1961–1964 Deputy Director Medical Services HQ BAOR.
7 Nov 1964 Temporary Brigadier L/RAMC.
1964–1966 Inspector Army Medical Services.
20 Aug 1965 Promoted Brigadier L/RAMC.
1966 Deputy Director Medical Services HQ 1 British Corps.
May 1966 Appointed an Officer (Brother) of the Most Venerable Order of the Hospital of St John of Jerusalem.
Dec 1966 Appointed Deputy Director of Medical Services Eastern Command with the rank of acting Major General in succession of Major General Donald Maurice Ahern.
5 Mar 1967 Promoted Major General. Appointed Deputy Director Medical Services HQ Eastern Command.
May 1968 Relinquished his appointment as DDMS Southern Command, but remained on full pay.
July 1968 Appointed Honorary Physician to the Queen in succession of Brigadier Mortimer Francis Howlett Kelleher who retired.
Sept 1968 Appointed Deputy Director General of Army Medical Services Ministry of Defence.
1968 Curator Royal Army Medical Corps Museum Mytchett Surrey. In 1977, he was succeeded as curator by Colonel Albert Victor Tennucci L/RAMC.
1 July 1969 Relinquished his appointment of Honorary Physician to the Queen on retirement. He was succeeded as Deputy Director General of Army Medical Services by Colonel Reginald John Gray.
Nov 1971 Appointed Colonel Commandant Royal Army Medical Corps.
1983 Resident at Gable House, Chequers Lane, North Crawley Bucks.