As a youth she wasn’t popular among her peers. “Fat and freckly with red hair and mad about horses,” remembers Clarissa Churchill. “We used to bully her.” Nancy Mitford was no kinder: “She was a red-headed bouncing little thing, regarded as a joke.” Among the debutantes of 1938 she did not shine, being neither rich nor beautiful. And yet despite this unpropitious beginning, Pamela Digby (later Churchill Harriman) would become one of the most influential, moneyed and talked-about women in postwar Anglo-American high society. Sonia Purnell explains how she did it in this sympathetic, well-researched, busily peopled but faintly exhausting biography, which will test even the keenest appetite for stories of ambition and the will to power.

Born into a privileged but cash-strapped family that sold up in Belgravia, London, and moved to Dorset, where her grandfather, the 10th Lord Digby, built a 50-room mansion without bathrooms (he considered them “disgusting”), she was an adventurous and energetic girl who so craved escape from loneliness that she gambled on marrying, aged 19, a man she had only known for two weeks. That her betrothed was Randolph Churchill, a bumptious brute much disliked in society, was both a personal catastrophe and the making of her. He had the pedigree, and provided an entrée to his parents, Winston and Clemmie, who took to Digby immediately. She was their mascot, a confidante and an initiate in “the low cunning of high politics”, often standing in for the fragile Clemmie and becoming a trusted member of Winston’s inner circle at Chequers and in the Whitehall war bunker. Her ascent came at a cost: Randolph, maddened with resentment, took his drinking and philandering to obnoxious new levels. A son, Winston, was the unlucky issue of the union.

The most astonishing passages of the book concern Churchill Harriman’s early role as an intelligence-gathering intermediary between the British war cabinet and the Americans, who were yet to commit to the fight against Germany. Handled like a bedroom spy by Max Beaverbrook, she seduced a number of high-profile bigwigs, at one point alternating her nights with the head of the US bomber command and the British chief of air staff, which sounds like the plot of a Preston Sturges comedy. Among her other conquests were dashing broadcaster Ed Murrow and Roosevelt’s trade envoy Averell Harriman, with whom she would reunite many years later. The confidence to play this grande horizontale came, as Purnell argues, from Churchill Harriman’s willingness to be alone. Most women were obliged to depend on men, but after Randolph she never would. War suspended the rules, and the trail of rivals and betrayed wives she left behind were not her concern.

While the end of the war brought relief, it also put the wind up her: she was 25, lonely and jobless. “I am afraid of not knowing what to do with life in peacetime.” It also leaves her biographer with another 300 pages and 50 years to fill, none of them nearly as compelling as 1939-45. In the event, Churchill Harriman continued her rampage through the bedrooms of rich and prominent men, but with the mood of peril and desperation gone the narrative wants for suspense. I began to miss somewhat the bulletins on the appalling Randolph – the Mark Thatcher of his day – whom even his father considered “perfectly useless”. Churchill Harriman cast around for a purpose, abandoning a diary job on Beaverbrook’s Evening Standard almost before it started, super-quick to realise how little journalism paid. Much better to fund a life from the plutocrats and playboys she picked up: learning sexual techniques from Prince Aly Khan, whizzing over to Cap Ferrat with Gianni Agnelli in his green Ferrari, helping a young Jack Kennedy search out his Irish cousins.

She made enemies of women, and not just the wives: Agnelli’s sisters thought her “a gold-digger”, and Brooke Hayward, having watched her Broadway producer father, Leland, succumb to her spell, described Churchill Harriman in her memoir as “vain, grasping and acquisitive”. On moving to Manhattan she became a society lady, magnificently dressed and “a-clank with gems” (Patrick Leigh Fermor). Truman Capote notoriously satirised her as Lady Ina Coolbirth, a ruthless femme fatale who had once been “had” by Joe Kennedy. When Leland died she became a “widow of opportunity” and was married within six months to her old flame, Averell. Wealth and a new life in Washington DC followed, though personal contentment remained elusive. Remorseful about her absent motherhood, she tried to win the affection of her son, Winston, with gifts of money and a private jet, but gallingly he always preferred his father.

Purnell makes a great deal of Churchill Harriman’s legendary charm, which finally landed her a job, aged 73, as Bill Clinton’s ambassador to France. She carried out the office honourably and won praise from the natives, too, albeit of a backhanded sort: Paris Match called her “a cross between Lady Hamilton and Moll Flanders”. The years of acquisitiveness caught up with her in the late 1980s when “financial distress” forced her to sell “her beloved Degas ballerina for $10m”. (We could all do with such distress.) Her troubles multiplied when she initially agreed to collaborate on her life story with a Time journalist, only to get cold feet and withdraw. A legal battle followed over a $300,000 payment the writer demanded for the work. Her lawyers eventually extricated her from the deal, and then presented her with their bill: $3m. She sold a home and a jet to raise cash.

The question: how much do you want to hear about enormously rich families suing one another for misuse of finances and then whining over how they’ve been cheated? Kingmaker, in the end, reminds me of a scene in a Saul Bellow story, Him With His Foot in His Mouth, when the narrator is seated at a charity dinner next to an old woman – a benefactor – who proceeds to explain at length how many millions she has spent on this or that cause, and how the fields are divided up among Rockefeller, Mellon, Rothschild et al. He notes: “The diamonds on her bosom lay like the Finger Lakes among their hills.” Presently the woman tells him that she’s planning to write her memoirs. Without missing a beat the narrator asks: “Will you use a typewriter or an adding machine?”