William Swainson Marshall was born in 1852 in Kaiwarra, near Wellington. He was a member of the Masonic craft, and took "a keen interest in all public matters". He owned a property called Te Hekenga in Hinau, near Pemberton, and was one of the pioneer settlers of the district. He grazed nearly 5000 sheep, and 200 shorthorn cattle. He was married to Elizabeth Hilda Swainson, and they had several children together.
Swainson was also the General Storekeeper and Postmaster of the Post Office Store in Pemberton, which was established in 1892.