Downing Street is globally recognised as the home of the British prime minister. It is also an early example of branding by the man who built Number 10, George Downing. Yet, as this lively and engaging biography tells us, this was probably the least interesting of Downing’s exploits. In Turncoat Dennis Sewell uncovers the life of this secretive spymaster who moved seamlessly between serving Oliver Cromwell in the 1650s and then Charles II in the 1660s. This is the first biography of Downing for a century (the last was John Beresford’s The Godfather of Downing Street in 1925) and it shines a light on a man who fleetingly appears in other histories, but who, as the author rightly argues, deserves to be better known. Was he, as Sewell asks, ‘the biggest scoundrel in Stuart England’?

Downing was born in Dublin in 1623 where his father Emanuel practised law; his mother, Lucy, was the sister of John Winthrop, the first governor of the Puritan colony of Massachusetts Bay. In 1638 the couple accepted Winthrop’s invitation to join him and in 1640 young George was one of the first intake at Harvard College. Unfortunately, Harvard’s master, Nathaniel Eaton, was a bully and sadist, who was rapidly sacked, but not before his wife had served the students pudding thickened with goat’s dung.

George Downing’s education had prepared him for a career in the congregational ministry and in 1645 he served as a ship’s chaplain in the Caribbean. When his ship sailed to England the following year Downing accepted a post as chaplain in the New Model Army regiment of John Okey, a man he would later betray. He then became chaplain to the regiment of Arthur Hesilrige, the parliamentarian governor of Newcastle upon Tyne. Downing was being noticed, and in 1649 Oliver Cromwell appointed him scoutmaster general of the English army in Scotland. He was no meek man of the cloth and was wounded three times during Cromwell’s ‘great victory’ at the Battle of Dunbar in 1650.

The main duty of a scoutmaster was to gather intelligence and Downing was in his element. He not only employed networks of spies wherever he was posted, but also turned key opponents of the Cromwellian regime into informers and agents. When the New Model Army entered Edinburgh after their victory at Dunbar, the governor of the castle, Walter Dundas, was remarkably reticent about meeting the English with force. Downing had already turned one of the governor’s servants into a double agent and may also have had Dundas in his pocket. The governor capitulated to the English and marched out of the castle with full military honours to a hearty dinner in Downing’s quarters.

Using his web of spies and double agents, Downing now kept the council of state in London reliably informed about the Scots’ secret negotiations for the return of Charles II to the throne. In 1654 he married Lady Frances Howard, the sister of the future earl of Carlisle, which opened up more political contacts. He represented Edinburgh in Cromwell’s first Parliament in 1654 and Carlisle in the Parliaments of 1656 and 1659. In 1657 Downing was posted as the English envoy to The Hague, where he and his spies were active in thwarting the designs of Charles II’s court in exile. The Restoration in May 1660 was a moment of acute anxiety for Downing, but he negotiated his continued employment with astonishing success. He was knighted by Charles II and kept his diplomatic post in The Hague.

Dennis Sewell admits that, as a good spymaster, Downing left no real evidence of how he achieved this remarkable transition, but provides some amusing speculation. Did Downing visit Charles II secretly dressed as a tramp to pledge his allegiance? Did he blackmail Tom Howard, his wife’s relative and a confidant of the king, to ease his path? Or did he act as a double agent himself by passing information to Charles II even before the Restoration had been signed, sealed, and delivered?

Downing certainly betrayed some of his former colleagues, and he pulled off a spectacular coup when he had three of the regicides, including John Okey, kidnapped in Amsterdam and returned to England as traitors. As Sewell pointedly observes, this was probably the first case of an extraordinary rendition (the abduction of an individual from one country to another by illicit means). On the scaffold, and with the spectre of a grisly death by hanging, quartering, and burning before him, Colonel Okey forgave his former chaplain ‘who pursued my life to the death’.

Downing was rewarded with a baronetcy and further lucrative offices, including secretary to the treasury. His New England background made him an ideal adviser to Charles on the Americas and the English acquisition of New York in 1664. As both his involvement in public affairs and his personal wealth grew, so too did resentment. He was suspected of corruption and it was reported that he kept six prostitutes in business, while Samuel Pepys, his former clerk, recorded that Downing was regarded as an ‘ungrateful villain’. In 1672 he was briefly imprisoned in the Tower of London for disobeying the king’s orders while on a diplomatic mission. Downing died in 1684 as a wealthy man and the greatest landowner in Cambridgeshire, though he did not live to see the completion of the development of the London address which bears his name.

Dennis Sewell’s book not only illuminates the shadowy world of 17th-century spycraft, it also traces the social, religious, and political networks that propelled Downing from his humble beginnings to positions of power. Yet we know little about Downing’s relationships with his family, nor if his career as a turncoat ever caused him qualms of conscience. Even the portrait of a self-confident, portly gentleman on the book’s cover does not give us any clues. The author does not analyse this arresting painting because its provenance is doubtful, the painter is unknown, and it may not even be Downing at all. The personal life of this spymaster and turncoat remains secretive to the very end.

• Turncoat: Roundhead to Royalist, the Double Life of Cromwell’s Spy 
  Dennis Sewell 
  Atlantic, 374pp, £25
  Buy from bookshop.org (affiliate link)

Jackie Eales is President of the British Association for Local History.