She was born in County Wicklow, and her family subsequently moved to Dublin. She married John Wyse Power and they had four children. Jennie herself ran a restaurant that became a meeting place for prominent Irish nationalist figures. She assumed a leadership role in many of the nationalist/feminist organisations of the time, from the Ladies Land League to Sinn Féin to the new feminist-separatist movement, Inghinidhe na hÉireann (Daughters of Ireland), to various suffrage organisations of the period.
Jennie was the first President of Cumann na mBan, an organisation founded in 1914 to assist the Irish Volunteers.
She was a part of that steady drift to 1916 and the Anglo-Irish war. Civil war followed the signing of the treaty with Great Britain. She supported the treaty and when the Cumann na mBan executive voted overwhelmingly against the treaty, she resigned.
In the Irish Free State, as a member of the new Senate, Jennie continued to be an advocate for women against the gender legislation of the 1920s and 1930s. First as a member of Cumann na nGaedhael, then as an Independent and later as a member of Fianna Fáil, she spoke out against discrimination. She saw it as particularly appalling that these measures were coming from men who had been given so much support from women in their fight for freedom.