William John Swainson was the eldest son of William Swainson, an English ornithologist and artist who emigrated to New Zealand in 1841. Born on June 18, 1824, he was the older brother of Edith Swainson, who later became Edith Halcombe when she married Arthur Halcombe.
Swainson married Emily Frances Broughton on August 17, 1865, and died some twenty years later on September 9, 1887.