Employed as clerk with A.B. Donaldson Ltd, Rarotonga, at time of enlistment. Enlisted 8 November 1915 and commenced duty 23 January 1916. Trained for New Zealand Medical Corps at Awapuni Camp, Palmerston North. Disembarked at Suez 13 March 1916 and embarked for France 9 April 1916. As he was proficient in Cook Island Māori language he was posted to New Zealand Pioneer Battalion as New Zealand Medical Corps attached for medical duties. Also employed in 3rd New Zealand Field Ambulance and New Zealand Division Rest Station. Attached to 3rd NZ Depot Unit of Supply at Bass-de-Radoub Base Supply Depot, Marseille, 5 February 1918 as interpreter for the transfer of Cook Islanders to Egypt. Embarked for Egypt on HMT Canberra 5 June 1918, arrived Alexandria 14 June 1918 and posted to Rarotongan Company 12 August 1918 as the Company medical orderly. He was detached from SS Malta in Fremantle, Australia, in 12 January 1919 as medical attendant to care and act as an interpreter for 16 soldiers (including 13 Cook Islanders) who were off-loaded and quarantined due to illness. Travelled from Perth to Melbourne by train 21 April 1919 and departed Sydney 7 May 1919. Discharged 9 July 1919 to live in Rarotonga. Was active in the Cook Islands RSA mid 1920s.

Biographical information from grand daughter Jane Horvath (née Shearman):
“I never knew my grandfather, but what I have learnt 68 years after his passing has given me an insight into the man he was. He grew up in Rarotonga to missionary parents who had sailed from London via New Zealand. I suspect he was a pacifist because of his family beliefs (London Missionary Society) and because my dad was sent to Whanganui from Rarotonga when he was 10 to attend a Quaker school. His call to duty to serve alongside the men he had grown up with would have been strong, so his contribution was to be a medic. He was the only child of Sophia and Thomas to go to war. I imagine that he would have seen the terror of war first hand, and although he never spoke of this, he would have carried the scars for the rest of his life. He married an English woman who he met in Australia on the homeward journey back to Rarotonga. I imagine that Percy was much like his only child, Ron, (my Dad) a gentle giant who was both generous and humble. At the heart of these two war veterans, was Rarotonga. The rock. Their hearts. For me it is important to acknowledge Percy's service alongside his care for his fellow Cook Islanders in lands so unfamiliar and far away from his beloved home. After 68 years, my grandfather, Percy Simeon Shearman, finally has a plaque to acknowledge his service during WW1. It's been an incredible journey researching a man who I never knew, but who shaped our family history from Rarotonga to Whanganui. Now his name is beside his son’s, and his story is told.
Kei wareware tatou - Lest We Forget.”