On 10 May 1894, Margot married Herbert Henry Asquith and became a "spur to his ambition". She brought him into the glittering social world which he had in no way experienced with his first wife, of whom it must be said she always spoke warmly. She also became the unenthusiastic stepmother of five children who were bemused by this creature, so different from their quiet mother. "She flashed into our lives like some dazzling bird of paradise, filling us with amazement, amusement, excitement, sometimes with a vague uneasiness as to what she might do next," wrote Violet Asquith. In 1908 when Asquith became Prime Minister, of the first brood of his children only Violet was still at home, and the two shared a deep interest in politics. In contrast, relations between stepmother and stepdaughter were frequently strained, prompting Asquith to write lamentingly of how the two were 'on terms of chronic misunderstanding.' It came as something of a relief to Margot when Violet married Maurice Bonham Carter, Asquith's Principal Private Secretary, in 1915. Violet in later life mellowed somewhat in her attitude to Margot, saying that she was grateful to her for her absolute devotion to Asquith.
Asquith bore five children, only two of whom survived infancy: Elizabeth in 1897, who married Prince Antoine Bibesco of Romania in 1919 and became a writer of some note, and Anthony in 1902, who became a leading English film director.