He served in the Australian Imperial Force during The Great War, from 4th February 1916 to returning to Australia from 6th September 1919. He was commissioned as a Second Lieutenant in the 44th Australian Infantry Battalion. The 44th Battalion was raised at Claremont, Western Australia in February 1916. It formed part of the 11th Brigade of the 3rd Australian Division, and soon became known as "Old Bill's Thousand" after its first commanding officer, Lieutenant Colonel Bill Mansbridge.
The battalion left Australia on 6th June and proceeded to Britain for further training. It arrived in France on 27th November and entered the front line trenches of the Western Front for the first time on 29th December. Eric was awarded the Military Cross, gazetted in Australia 17th June 1919 on page 1012 at position 4.
He married Elizabeth Currie in 1924 at Perth, Western Australia. 
Marriage registration : (1841-1936) 
Surname Given Names Sex | Spouse Surname | Given Names Sex | District | Number | Year
Mckenzie Eric G H Male | Currie | Elizabeth M Female | Perth | 823 | 1924
His command of the 13th Infantry Brigade of the Australian Army during the Second World War was the longest command held by any commander; from June 1940 to May 1945, when he relinquished command due to illness. From 23rd July 1942 to 15th October 1945 he was a member of the Second Australian Imperial Force. Before and after that period he was a member of the Australian Army militia.
Eric passed away during 1957 in Perth,  and his ashes were interred at Karrakatta, Cemetery. 
Death registration : (1841-1971) 
Surname Given Names Sex | Age | Father | Mother | District | Number | and Year
Mckenzie Eric George Henderson | Male | 61 | George H | Agnes W | Perth | 2302 | 1957
Categories: Scouts Australia | AIF, 44th Australian Infantry Battalion | Headquarters 13th Australian Infantry Brigade, World War II | Scout Leaders WA1908 | Karrakatta Cemetery, Karrakatta, Western Australia | Anzacs, World War One