William Lillie was born in Rotherhithe, Kent on 24 July 1896, the son of Albert Frederick Lillie and Hannah Lillie (née Meadows). He was appointed Probationary Midshipman RNR on 15 September 1913 and was appointed to the cruiser HMS PRINCESS ROYAL on the same date. He was appointed as Midshipman, Royal Navy, on 23 March 1915. Whilst serving in HMS PRINCESS ROYAL he took part in the action at the Heligoland Bight on 28 August 1914 and at the Battle of the Dogger Bank on 24 January 1915. On 20 April 1916 he was appointed to the cruiser HMS PHAETON. On 4 May 1916, HMS PHAETON, in company with HMS GALATEA, sighted and engaged the Zeppelin L7, damaging L7 such that it was brought down in the sea close to submarine HMS E31. The submarine completed the destruction of L7 and rescued seven survivors. William Lillie was onboard HMS PHAETON at the Battle of Jutland on 31 May/1 June 1916 and promoted to Sub Lieutenant on 15 November 1916.

On 7 July 1917, he was appointed to HMS DOLPHIN for submarine training. William Lillie was appointed to the Submarine Depot Ship HMS MAIDSTONE at Harwich for submarine duties to date 24 September 1917. He was next appointed to HMS C1 as First Lieutenant on 1 December 1917. He was promoted to Acting Lieutenant on 15 January 1918 and appointed to HMS E34 as First Lieutenant on 1 April 1918. E34 was lost with all hands in the North Sea on 20 July 1918. The cause of the loss has not yet been established. He was awarded the DSC for services in the torpedoing and sinking of an enemy submarine on the 10 May 1918.

William Lillie’s body was washed ashore after his death, and he is buried in the Hamburg Cemetery in Grave No II.H.13.