In 1912, Montagu accompanied the prime minister on holiday in Sicily. H. H. Asquith brought along his daughter Violet, and she in turn brought her friend Venetia Stanley, daughter of Edward Stanley, 4th Baron Stanley of Alderley. It appears that during this holiday, both men fell in love with Stanley.
During the next three years, Asquith wrote more and more frequently to Venetia. At the same time, Montagu was attempting to court her, unsuccessfully proposing marriage in 1913. She liked him but did not reciprocate his love. Also, Montagu had to marry within his Jewish faith in order to keep his inheritance. Although Stanley was from a freethinking family and was not a devout Anglican, conversion to Judaism seemed too great a barrier.
Although Stanley was intelligent, well-read and keenly interested in politics, she apparently felt overwhelmed by Asquith's demands. As a result, she finally accepted Montagu's proposal on 28 April 1915. She converted to Judaism, and the couple were wed on 26 July 1915.
A 2012 book argues that the marriage was one of social convenience to cover both Montagu's homosexuality and Venetia's earlier affair with Asquith. Conspiracy of Secrets suggests that this affair had resulted in the 1912 birth of an illegitimate child, Louis Stanley, who grew up to run British Racing Motors.
The marriage was unhappy and she had several affairs, including one with the press magnate Lord Beaverbrook. In 1923 a child was born: legally and socially Judith was Montagu's daughter, but she was probably fathered by William Humble Eric Ward, then Viscount Ednam and later 3rd Earl of Dudley. She grew up to befriend Princess Margaret during World War II and marry the American photographer Milton Gendel, with whom she created an artistic salon in Italy.
Despite his wife's affairs, Montagu's marriage lasted until his premature death in 1924.