The son of Thomas "The Settler" Dewey & his wife Frances, later the wife of George Phelps. Dewey married on 6 Nov 1662 at Northampton, MA to Hepzibah Lyman the daughter of Richard & Hepzibah Ford Lyman. The Deweys located at Northampton, MA about 1663 where Josiah learned the carpenter's trade and became a freeman in 1666. During King Philip's War he was a Sargent of the Guard at Westfield, MA.

He was a Quaker and in his religious discovery he says; "When I was entred into a married state I saw myself now under former ingagemnts of attending heart-searching & hearing Mr. Mather on the hearts hardness assert that there was no plague like unto that, I was affrighted there at & soon after hearing Mr. Eliot (now of Gilford) on a lecture sermon was so awakened as to resolve no longer to delay but to fall to search my own heart. But I found it hard & difficult work to keep my mind to it & sometimes I found that my heart would slip from ye work almost as soon as I was at it. So that I could find little rest."

He seems to have found that rest as he became a Deacon in the church at Westfield. In 1696 he moved to Lebanon, CT where he helped found the town, and where his name appears often in the early records.