The earliest reported case in which he was engaged was tried in the Queen's Bench in Hilary term 1597–8, when he acted as junior to the attorney-general, Coke.

Crewe's professional reputation was somewhat damaged by the Leicester boy Witch Trials, where he sat as an extra judge of assize.

Nine women were hanged on the evidence of a young boy called John Smith whom Crewe, and his colleague Sir Humphrey Winch, found entirely credible, but whom King James soon after declared to be a fraud.

The same year Crewe prosecuted Sir Francis Mitchell for alleged corrupt practices in executing 'the commission concerning gold and silver thread,' conducted the impeachment of Sir John Bennet, judge of the Prerogative court, for corruption in his office, and materially contributed to the settlement of an important point in the law of impeachment.

The Crewe family is said to be among the most ancient in the kingdom, a fact the importance of which is not likely to have been underrated by Sir Ranulph, if we may judge by his eloquent prologue to the Oxford peerage case, decided 1625, which is one of the few passages of really fine prose to be found in the Law Reports.

"Time" he said, "hath his revolutions, and there must be an end to all temporal things, finis rerum"... "Where," he asks, "is Bohun, where's Mowbray, where's Mortimer?

After the impeachment in 1641 of the judges who had affirmed the legality of Ship money, Denzil Holles moved the House of Lords to petition the king to compensate Crewe, who seems to have passed the rest of his days in retirement, partly in London, and partly at his seat, Crewe Hall, Barthomley, Cheshire, built by him upon an estate said to have belonged to his ancestors, which he purchased from Coke in 1608.

A letter from Crewe to Sir Richard Browne at Paris, under date 10 April 1644, describing the growing exasperation of 'this plus quam civile bellum,' as he called it, and the devastation of the country, is preserved in the British Museum, and is printed in the Fairfax Correspondence.