His elder brother, Robert Duncon was also born in Mendlesham, a year earlier in 1607.
He was ordered to march with the king's forces against the Scots; but he was allowed, after some troublesome negotiations, to hire a substitute.
When the English Civil War broke out, he as well as his father and father-in-law, aided the parliament with many contributions, by raising troops (which brought him into direct communication with Cromwell), and by acting as high collector of assessments until 1651.
Duncon wrote:[2] The chief end of these tracts is (besides the recital of the author's sacrifices for the Commonwealth) towards the "settling of peacemakers in every city and county of this nation".
These peacemakers were to be the "most understanding plain honest-harted men" that the people of the district could find.