His diaries of proceedings in the House of Commons are an important source for historians of parliamentary history during the period when he was a member, and Sir John Neale draws heavily upon them in his ground-breaking two-volume study of Elizabeth I and Her Parliaments (1953–1957).

[1][3] Cromwell sat in five successive Parliaments between 1571 and 1589 during the reign of Queen Elizabeth I: for Fowey (1571), Bodmin (1572–1581), Preston (1584–1585) and Grampound (1586-7 and 1589).

His return at Preston in 1584 may be accounted for by the patronage of Sir Ralph Sadler, chancellor of the Duchy of Lancaster, who was brought up in the household of Cromwell's grandfather.

Neale described him as the model type of parliamentarian, deeply versed in the history and procedure of the institution, though lacking in historical perspective; eminently responsible, but fearless in defence of liberty.

He died between February 1610 and April 1611, leaving money and property to his wife "who has always been a most loving wife... and hath besides endured many griefs and sorrows for my sake", to his children, subject to their good behaviour and money to the poor of Great Risborough, Norfolk, and to the poor of the parish where he died.