He enlisted at Adelaide on 19 January 1916, and almost immediately was sent to non-commissioned officers school and provisionally promoted to Sergeant. After spending time training reinforcements at Mitcham Camp he was sent to attend the 3rd Officer Training School at located at Duntroon in the Australian Capital Territory. Whilst he was at Duntroon he received news that his younger brother Lance Corporal Hugh Phillips Leak of the 10th Battalion had been killed in action at Pozieres. Llewellyn graduated with the rest of No.5 Platoon in August 1916 and returned to Adelaide.
On 1 October 1916 he was commissioned as a Second Lieutenant, and spent the next year or so again training reinforcements at Mitcham Camp. On 9 December 1916 Llewellyn married Stella Gladys Evans at his grandparents house at Glenelg. Stella was the fourth daughter of David Evans of Upper Sturt, a sister of Douglas Evans (also on this blog) and the couple lived at Upper Sturt. Llewellyn entrained for Melbourne on 27 October 1917, and embarked with the rest of the 20th reinforcements to the 27the Battalion on the 'Aeneas' on 30 October 1917. Whilst he was at sea on 23 December 1917, his wife Stella gave birth to a son, Weston Hugh Leak.
After some more training in England, he crossed the Channel to France and finally joined his unit on 10 April 1918, taking up over command of 13 Platoon, 'Don' Company, 27th Battalion which was resting near Albert, France. Cecil Oswald, whom he had gone through officer training with back in Australia, joined the battalion the same day. The Battalion spent the rest of April and May in fairly quiet sectors, rotating through frontline, support and reserve areas.
The following month the battalion was part of the successful 7th Brigade attack at Morlancourt on 10 June, when the unit killed over 130 of the enemy, and captured one officer and 80 men, as well as 13 machineguns and two trench mortars. On 8 July, Llewellyn was promoted to Lieutenant, and at the end of the month he was sent to the Australian Corps Bombing (Grenade) School to attend a short training course.
On 2 September 1918 the 27th Battalion was ordered to capture the villages of Allaines and Haut Allaines near Peronne. During the attack, 'Don' Company was the leading right hand company, and German machineguns on high ground to the right of the advancing troops caused a large number of casualties, including all but two of the officers of 'Don' Company. Llewellyn Leak was one of the officers wounded, in his case in the chest and throat. He was evacuated first to the 6th Field Ambulance then the 5th Casualty Clearing Station. His condition on arrival at the casualty clearing station late on 2 September was described as 'very grave'. He hung on for two more days, but died of his wounds at 7.15pm on 4 September 1918. Soldiers that had served under his command later described him as a 'brave soldier' who was 'loved and respected by his men, with whom he was very popular'.
He was buried in the Proyart communal cemetery extension on 5 September 1918. A few years later his remains were exhumed and laid to rest in the Heath Cemetery at Harbonnieres.
Stella Leak died in 1976 at the age of 81. During the Second World War, Llewellyn and Stella's son Weston served as a Lieutenant with the 2nd/6th Field Ambulance.
Llewellyn Leak's name is inscribed on the State National War Memorial and honour boards in the Glenelg Town Hall, North Adelaide Cycling Club, Adelaide Railway Station and the Upper Sturt Methodist (now Uniting) Church.