Seaton Milligan was born in Cloncur, County Tyrone, and was educated locally. Through his business travels he became familiar with Ulster and Connaught. He was interested in Irish antiquities and in 1884 became a member of the Royal Society of Antiquaries and a Fellow in 1888. He contributed many papers to the journal of that society. With his daughter Alice, he published Glimpses of Erinn and also a guide to Tyrone and Fermanagh.