LONDON — Biographer Anne Sebba remembers the shocked reactions of some fellow Englishmen about the roots of her latest subject, Jennie Churchill.

“I told them Winston Churchill had an American-born mother, but a lot of people didn’t believe me. They said it wasn’t possible,” Sebba recalls with a laugh.

Indeed, while it’s common knowledge that England’s greatest 20th-century statesman came from the grand Spencer-Churchill family (Winston wisely dropped the double-barreled name so as not to appear too posh), few realize his mother was a nouveau riche, fashion-mad and often pushy Brooklyn girl called Jennie Jerome.

While the Spencer-Churchills were — and still are — among England’s foremost clans with their family seat at Blenheim Palace, one of the country’s grandest stately homes, the Jeromes couldn’t even get invited to one of Mrs. William Astor’s famous private balls in New York. But that didn’t stop Jennie Jerome.

In her book, “Jennie Churchill: Winston’s American Mother” (John Murray), Sebba argues that it was Jennie’s American pluck — and stage-mother personality — that helped make Churchill the great statesman he became. And while the passionately written book — which won rave reviews in England — is a biography, it’s really the story of a dynamic mother-son relationship.

“She was never a cuddly, nurturing mother — she was ambitious, well-educated, knew her ground and her power over Winston. She believed in her son’s destiny,” says Sebba during an interview at the English Speaking Union in Mayfair. “She may not have been the best mother when Winston was young, but she was the mother he needed. All of her ambitions were focused on him.”

Born in 1854 and raised in Brooklyn, the beautiful, dark-eyed Jennie was educated mostly in Paris. Her upwardly mobile family had moved there to escape snobby, claustrophobic New York high society and try their luck at the more democratic court of Empress Eugenie, the wife of Napoleon III.

“In those days, Paris was the place to break through — Empress Eugenie loved Americans and accepted them,” says Sebba. During her years in Paris, Jennie was taught to play the piano to concert standard, learned about world affairs — and the merits of discretion — and developed a penchant for shopping, happily spending her father’s cash on dresses at couturier Worth.

This story first appeared in the December 28, 2007 issue of WWD. Subscribe Today.

“Her clothes were always remarked upon, and she understood the power of retail therapy. When she was living in England, and feeling down, she’d travel to Paris to shop and that would buck up her spirits,” says Sebba.

Jennie met her future husband, Lord Randolph Churchill, in the summer of 1873 while her family was vacationing on the Isle of Wight, off the southwest coast of England. She and her sister had been invited to a ball in honor of the Grand Duke and Grand Duchess Cesarevitch, and Jennie Jerome and Churchill fell instantly in love on the dance floor.

They married the next April and Winston was born “prematurely” in November. Sebba is convinced the future prime minister was conceived before the marriage, but says the incident only testifies to his parents’ passionate relationship, and Jennie’s self-assurance in bedding her fiancé at a time when most women rarely took such risks.

Sebba says so many of Jennie’s thwarted desires for Randolph, the Duke of Marlborough’s third-born son, were channeled into young Winston. “She had hoped that Randolph would become prime minister — it was something that was being talked about,” Sebba says. While Randolph had served as Chancellor of the Exchequer (the senior finance minister in the British government) and leader of the House of Commons, he was never destined to hold the prime minister’s office and died of syphilis at age 45.

Jennie would go on to marry twice more — to men young enough to be her son — and she certainly had her share of extramarital love affairs before she died in 1921. Sebba, however, argues that Winston was always “the number-one man in her life,” and she, in turn, was his staunchest supporter. “She was his confidante, literary agent and editor, and chief networker,” says the author.

For instance, while visiting soldiers injured during the various wars, instead of offering sweet words of comfort like most volunteers, Jennie urged them to “Vote for Dear Winston” in upcoming parliamentary elections.

Yet the mother never did get to see her son become prime minister in 1940. “It was a tragedy — but she knew it was his destiny to take on that role. She had drama in her blood,” says Sebba.

That sense of drama followed her everywhere: Jennie died, “aged 67, and in her prime” according to Sebba, after breaking her ankle in a new pair of high heels. The fall brought on gangrene, followed by a leg amputation that resulted in a burst artery.

Sebba, whose previous subjects include Mother Teresa and Laura Ashley — “I like writing about strong-minded women,” she says — doesn’t know who will strike her fancy next. “Jennie is a hard act to follow — and I’m still completely obsessed and engaged with her.”