She was born to lapsed Quaker parents, Lewis Jacob and Henrietta Harvey, in Waterford, where she lived until 1920.
Her first novel was called Callaghan and was published in 1920. She opposed the Anglo-Irish Treaty of 1921 and was especially involved in left-wing and republican organisations in the 1920s and 1930s. She was imprisoned in Mountjoy Jail during the Irish Civil War.
She was involved in the Women"s International League for Peace and Freedom and later in the Irish Housewives Association.
In the 1920s and 1930s she was involved in a relationship with fellow republican Frank Ryan. She lived in the Rathmines area of Dublin from at least 1942, firstly in Belgrave Square.
She died in 1960. Rosamond Jacob kept a diary almost all of her life, and there are 171 of these diaries among her literary and political papers held in the National Library of Ireland.
She was a lifelong activist for suffragist, republican and socialist causes and a writer of fiction.
She played a leading role in the political campaign to secure Ryan’s freedom from Nationalist Spain, and later worked to defend his reputation after news of his death in Nazi Germany became known.
She was also a member of Cumann na mBan, the Gaelic League and the Irish Women"s Franchise League.