She was born at 12 John Street, Berkeley Square, Mayfair, in London, the eldest daughter and heiress of Colonel Frederick Hambleton Custance, who was a wealthy and distinguished soldier in the British army. Custance spent the much of her childhood at Weston Old Hall in Norfolk, the family seat.
Custance joined the London literary circle around such figures as Oscar Wilde, Aubrey Beardsley, Ernest Dowson and John Gray in about 1890 when she was only 16. At this time she became infatuated with the poet John Gray and wrote some of her first poetry about him. Heavily influenced by French poets such as Verlaine and Rimbaud and by the decadent mood of that period, she quickly rose to prominence as a poet. In 1901 she became involved in a relationship with the overtly lesbian writer Natalie Clifford Barney in Paris, which Barney later included in her memoirs. Barney, and a friend of hers, Renée Vivien, were keen to win Custance as a partner, and indeed Custance remained on close terms with Barney for years. Custance and Barney exchanged love poems, including Custance's poems 'The White Witch'. Vivien's roman à clef A Woman Appeared to Me (1904) also recounts her brief relationship with Custance.
During her brief affair with Barney, Custance also instigated a courtship with Lord Alfred Douglas by writing to him admiringly in June 1901, six months after the death of Oscar Wilde. The two corresponded under the nicknames of the 'Prince' (for Douglas) and 'Princess' or 'Page' for Custance.
However, in late 1901, in an odd turn of events, Custance became engaged to George Montagu, who had been at school with Douglas. It was a short engagement because when Douglas returned from a trip to the USA (where, as he had written to her teasingly, he was looking for a rich heiress to marry) the two of them ran away and married each other on 4 March 1902. Custance's father did not approve of Douglas. They had one child, Raymond Wilfred Sholto Douglas, born on 17 November 1902. The marriage was stormy after Douglas became a Roman Catholic in 1911. They began to live apart in 1913, after the couple lost a custody battle for their only child to Custance's father.
In 1913 Douglas was charged with libelling his father-in-law who had always disapproved of him and seems to have been a major reason for strain on their marriage. The couple again lived together for a time in the 1920s after Olive also converted to Catholicism in 1917.