"Jeanette Jerome" Long considered one of the most beautiful women of the time, she was married for the first time on April 15, 1874, aged 20, at the British Embassy in Paris, to Lord Randolph Churchill, the third son of John Winston Spencer-Churchill, 7th Duke of Marlborough and Lady Frances Anne Emily Vane. Although the couple became engaged within three days of their initial meeting, the marriage was delayed for months while their parents argued over settlements. By this marriage, she was properly known as Lady Randolph Churchill and would have been referred to in conversation as Lady Randolph.
The Churchill's had two sons. Winston Leonard Spencer-Churchill (30 November 1874 – 24 January 1965), the future prime minister, was born less than eight months after the marriage. According to his biographer William Manchester, Winston was most likely conceived before the marriage, rather than born prematurely. (A recent biography has stated that he was born two months prematurely after Lady Randolph "had a fall.") When asked about the circumstances of his birth, he would reply, "Although present on the occasion, I have no clear recollection of the events leading up to it." Lady Randolph's sisters believed that the biological father of the second son, John Strange "Jack" Spencer-Churchill (4 February 1880 – 23 February 1947) was Evelyn Boscawen, 7th Viscount Falmouth.
Lady Randolph is believed to have had numerous lovers during her marriage, including Karl Kinsky, the Prince of Wales (later King Edward VII of the United Kingdom) and Herbert von Bismarck.
As was the custom of the day, Lady Randolph played a limited role in her sons' upbringing, relying largely upon nannies, especially Elizabeth Everest. Winston worshiped his mother, writing her numerous letters during his time at school and begging her to visit him, which, however, she rarely did. He wrote about her in My Early Life: 'She shone for me like the evening star. I loved her dearly - but at a distance'. After he became an adult, they became good friends and strong allies, to the point where Winston regarded her almost as a political mentor, more as a big sister than as a mother. She was well-respected and influential in the highest British social and political circles. She was said to be intelligent, witty, and quick to laughter. It was said that Queen Alexandra especially enjoyed her company, despite the fact that Jennie had been involved in an affair with her husband, King Edward VII, a fact that was well known by Alexandra. Through her family contacts and her extramarital romantic relationships, Jennie greatly helped Lord Randolph's early career, as well as that of her son Winston. In 1909 when American impresario Charles Frohman became sole manager of the The Globe Theatre, the first production was His Borrowed Plumes, written by Lady Randolph Churchill. Although Mrs. Patrick Campbell produced and took the lead role in the play, it was a commercial failure. It was at this point that Mrs. Campbell began an affair with Lady Randolph's then husband, George Cornwallis-West.