Nancy Cunard introduced Warner to Mary Valentine Ackland and the two women lived together for the rest of their lives. Warner's first novel, Lolly Willowes, was published in 1926. This was followed by the novels Mr. Fortune's Maggot (1927) and The True Heart (1929).
Warner was the co-editor of the 10-volume Tudor Church Music (1923-29). Warner also wrote poetry and in 1933 published Whether A Dove or Seagull with Ackland. A regular contributor to New Yorker, Warner's poetry was praised by Alfred Edward Housman and William Butler Yeats and Louis Untermeyer compared her to Thomas Hardy.
Warner, was a leading member of the Communist Party of Great Britain in Dorset. She also gave support to the Spanish Medical Aid Committee, an organization that had been set-up by the Socialist Medical Association and other progressive groups. Other members included Leah Manning, Isabel Brown, George Jeger, Lord Faringdon, Arthur Greenwood, Tom Mann, Ben Tillett, Harry Pollitt, Hugh O'Donnell, Mary Redfern Davies and Isobel Brown. Soon afterwards Kenneth Sinclair Loutit was appointed Administrator of the Field Unit that was to be sent to Spain.
In September 1936 Warner and Mary Valentine Acklandwent to Spain and provided help to the British Medical Aid Committee supporting the Republican Army. They were inspired by the revolutionary atmosphere they found in Barcelona. Warner wrote: "I don't think I have ever met so many congenial people in the whole of my life." However, the two women wrote a letter of complaint to Harry Pollitt about the "failure to adopt a satisfactory social attitude" by some members of the Communist Party of Great Britain.
However, Warner and Ackland went along with the Communist Party (PCE) repression of Worker's Party (POUM) in Barcelona. In The Daily Worker Ackland argued that the action had been justified because of "POUM's utopian non-authoritarian politics spelled certain death to the Republic pitched against the military strength of the Fascist powers."
On her return to Dorset Warner wrote an article for The Left Review about the way the Catholic Church oppressed the Spanish people. In a letter to her friend Elizabeth Wade she made the same point: "I have never seen churches so heavy and hulking and bullying, one can see at a glance that they have always been reactionary fortresses. I did not find a single person of any class who resented their being gutted, though we did find two domestic servants... who felt a certain uneasiness about it, as though God might pop out of those ruined choirs and grab them by the scruff. The not being able to read and write is the crux. A people naturally intellectual, and with a long standard of culture, have thrown off the taskmasters who enforced ignorance on them."
As Angela Jackson pointed out in British Women and the Spanish Civil War (2002): "They (Ackland and Warner) were involved in the founding of the local Left Book Club and Sylvia was secretary of the Dorset Peace Council. They were ardently committed to the cause of the Spanish Republic, campaigning and fund-raising in a whirl of breathless non-stop activity."
The following year the two women went to Madrid and Valencia as part of the British delegation to the Second Congress of the International Association of Writers for the Defence of Culture. Twenty-six countries participated in the conference. Warner wrote about it in an article published in Time and Tide in August 1937: "We learned to hear ourselves spoken of as los intelectuales without dreading words usually so dubious in good intent, without feeling the usual embarrassment and defiant shrinking."
Other novels by Warner include Summer Will Show (1936), After the Death of Don Juan (1938), The Corner That Held Them (1948) and The Flint Anchor (1954). During her career Warner published seven novels, ten volumes of short stories, five volumes of poetry and a biography of T. H. White.
Warner eventually left the Communist Party of Great Britain. The historian, Angela Jackson, argues: "Sylvia Townsend Warner allowed her membership of the Party to lapse during the 1950s. Along with other writers on the Left, she became a literary casualty of the Cold War." When she was asked why she had ceased to be active in politics: "We had fought, we had retreated, we were betrayed and now we were misrepresented."
Sylvia Townsend Warner died in 1978.