He was the son of Walter James Barrett-Lennard and Caroline Dormer. He was the grandson of Sir Thomas Barrett-Lennard, 1st Baronet. He married Mary Emma Gardiner in 1887; they had one son, the Rev. Dacre Bartett-Lennard.
He was lieutenant in the Reserve , but resigned this commission on 3 March 1900. He fought in the Second Boer War and the First World War (9th Bn, Essex Regiment). He was appointed CBE in 1919 and awarded the French Legion of Honour.
- ‘BARRETT-LENNARD, Lt-Col John’, Who Was Who, A & C Black, an imprint of Bloomsbury Publishing plc, 1920–2007; online edn, Oxford University Press, Dec 2007 accessed 3 Feb 2014
- The Times (London, England), Wednesday, Apr 03, 1935; pg. 16; Issue 47029. (275 words)
- "No. 27170". The London Gazette. 2 March 1900. p. 1436.