Barrett-Lennard was the son of Captain Thomas George Barrett-Lennard and Edith Mackenzie. He married Winifrede Mignon Berlyn in 1916. They had one son, Hugh, who subsequently inherited the Barrett-Lennard baronetcy.
Barret-Lennard was appointed as one of the Puisne Judges of the Supreme Court of the Gold Coast (later Ghana) in 1913. He was subsequently a judge in the Straits Settlement. He was appointed Chief Justice of Jamaica in 1925 and was knighted the following year. As Chief Justice, in 1929, he ordered the confiscation of the property of the Universal Negro Improvement Association (UNIA) which had been founded by black activist, Marcus Garvey. He retired in 1932 and after retiring, he claimed his retirement was forced on him by ill health that resulted from having been poisoned. He returned to London, becoming a lecturer at Birkbeck College and wrote a paper on colonial law published in the Transactions of the Grotius Society.
- ‘BARRETT-LENNARD, Sir Fiennes Cecil Arthur’, Who Was Who, A & C Black, an imprint of Bloomsbury Publishing plc, 1920–2008; online edn, Oxford University Press, Dec 2012 ; online edn, Nov 2012 accessed 19 June 2013
- The Times, Monday, 28 Jan 1963; pg. 12; Issue 55610; col C Sir Fiennes Barrett-Lennard
- Chelmsford Chronicle, 8 August 1913
- The Straits Times, 25 September 1932
- The Singapore Free Press 17 October 1932
- Article in Jamaican Journal
- Western Daily Press, 9 August 1932
- Straight's Times 4 October 1932
- Some aspects of colonial law