Cooper was then still a minor, but his ward-ship was acquired by an uncle for £324, after a sweetener of £600 was paid to Robert Cecil, 1st Earl of Salisbury, who was at the time the Lord High Treasurer.

Cooper's connection with Dorset dated from his marriage to Sir Anthony Ashley's only child, but initially he played little part in local affairs.

He is not known to have contributed to debate during the first Caroline Parliament, and was appointed only to legislative committees concerned with larceny, benefit of clergy, and concealed Crown lands (25 June).

Despite his hostility to the doctrines, practices, and ceremonies associated with the Pope or the papal system, Cooper displayed no obvious puritan leanings, being ‘of an easy and an affable nature’ and a compulsive gambler.

His first wife died of smallpox in 1628, shortly after succeeding to the Ashley estate, and Cooper, who was reputedly "very lovely and graceful both in face and person".