Today, Sir George Downing is remembered, if at all, for the street and row of brick houses in London that bear his name. This address—the home to Britain’s prime ministers and the site where the cabinet meets—was meant to be a place where the organs of the British state are held accountable to the will of the people. It is a great irony then, and perhaps mildly appropriate, that Downing, the man who lived in the 17th century, was as cynical and treacherous a figure as the era ever produced. Winston Churchill, nearly three centuries later, would remember him unflatteringly as “a profiteering contractor.”

All but forgotten are the elaborate contours of Downing’s career: spy, diplomat, financier, parliamentarian, early New Englander, and member of Harvard’s first graduating class. Dennis Sewell’s biography, Cromwell’s Spy: From the American Colonies to the English Civil War: The Life of George Downing, accomplishes the rare feat of summoning a figure both long overlooked and profoundly relevant.

Downing’s life is the story of modern Britain’s painful birth—a damp North Atlantic island sorting its own domestic affairs, consolidating global influence, and teetering precariously on the cusp of empire. Born in Dublin in 1623, Downing was fated to become a New Englander. In the late 1630s, his family sailed among the 20,000 Puritans who went to the Massachusetts Bay Colony, a less deadly enterprise than that of the first pilgrims who founded Plymouth and perished in large numbers. Downing’s maternal uncle was John Winthrop, governor of Massachusetts Bay, and his father, Emmanuel Downing, was a notable barrister.

Downing attended Harvard in 1638, not long after it was established by scholars from Cambridge and a bequest from the eponymous John Harvard. As Sewell writes: “Harvard was founded by Puritans in the pious hope that it would turn out saints, but in George Downing it apparently summoned a demon.” The college was notoriously harsh during his time: Nathaniel Eaton, master of the college and a “brutal sadist,” beat pupils well beyond what seemed reasonable. His wife had a habit of “thickening the hasty pudding” with goat’s dung. Undergraduates took their first lectures at 7 a.m. and were required to converse in Latin.

By 1642, Downing was second in line among Harvard’s first graduates—seven battle-hardened polyglots who studied Greek, Latin, Hebrew, Chaldean, and Syriac. The order of precedence was likely determined by the faculty’s estimate of “to what degree each would shine in their future careers.” After serving as Harvard’s first tutor, Downing went to the Caribbean to preach before sailing for England during its revolutionary turmoil.

Sewell struggles to deliver on one of his core assumptions: that Downing was a man of singular treachery so notable it endured centuries after his death. Yet Downing was exceptionally well-suited to his violent and dangerous times. The book reveals many turncoats and traitors, but Downing may have outdone them in his cleverness—though perhaps not in his wickedness.

In England, Downing impressed Oliver Cromwell enough to become head of military intelligence. On undercover missions in Scotland, he proved particularly effective. When the New Model Army marched into Edinburgh, Downing’s agents were already holed up in the castle with the last defenders—a prime position from which they facilitated the handover of the city. He was later trusted to meet with Europe’s leading statesmen, negotiate backroom deals, and gather intelligence.

By 1660, after Charles II restored the monarchy, Downing managed not only to survive but retain prominent roles as envoy to The Hague and Teller of the Exchequer. In some instances, it was Downing himself who hunted down dozens of republicans and conspirators—former friends and allies executed for regicide in Charles I’s death—and secured their rendition from Holland. He also played a key role in helping secure New Amsterdam (now New York) from the Dutch, an event that left a second Downing Street running through Greenwich Village.

Downing emerges as a man riven with contradictions: a Puritan by upbringing yet deeply amoral; fantastically wealthy but incredibly tight-fisted. In his old New England stomping grounds, “doing a George Downing” became synonymous with treachery. President John Adams, writing two centuries after Downing’s death, recalled him as both a “dog” and a “scoundrel.” Yet Downing’s pragmatism solved many complicated problems for his country.

Sewell’s biography, though not a full portrait of Downing, is a gripping and impeccably literate study of one of England’s and New England’s forgotten sons. As the book suggests: “The civil war had a way of making double dealers out of even the most honest men and women.”