Born Carrie Katherine Richards in Ottawa County, Kansas, O'Hare briefly worked as a teacher in Nebraska before becoming an apprentice machinist in her native Kansas, where she was drawn into socialist politics and married fellow socialist Frank P. O'Hare. She unsuccessfully ran as a candidate for the United States Congress in Kansas on the Socialist ticket in 1910.
As editor of the National Rip-Saw, a St. Louis-based socialist journal in the 1910s, O'Hare championed reforms in favor of the working class and toured the country as an orator. After America's entry into World War I in 1917, O'Hare led the Socialist Party's Committee on War and Militarism. For giving an anti-war speech in Bowman, North Dakota, O'Hare was arrested and taken to prison by federal authorities for violating the Espionage Act of 1917, an act criminalizing interference with recruitment and enlistment of military personnel. With no federal penitentiaries for women existing at the time, she was delivered to Missouri State Penitentiary on a five-year sentence in 1919, but in 1920 was pardoned by President Warren Harding after a nationwide campaign to secure her release. In prison, O'Hare met the anarchists Emma Goldman and Gabriella Segata Antolini, and worked with them to improve prison conditions.
O'Hare, unlike Socialist Party leader Eugene V. Debs and other prominent socialists at the time, was a supporter of racial segregation, and penned a 1912 pamphlet titled "'Nigger' Equality", which attempted to appeal to Southern voters .
Kate O'Hare divorced Frank O'Hare in June 1928 and married the engineer and businessman Charles C. Cunningham in California in November the same year. Despite her continued involvment in politics, much of O'Hare's prominence gradually faded. O'Hare worked on behalf of Upton Sinclair's Socialist campaign in the 1934 California gubernatorial election, and briefly served on the staff of Wisconsin Progressive Party politician Thomas R. Amlie in 1937-1938. Esteemed as a prison reformer, she served as an assistant director of the California Department of Penology in 1939-1940.
O'Hare died in Benicia, California, on 10 January 1948.
- O’Hare, Kate Richards. The Sorrows of Cupid. St. Louis: National Rip-Saw, 1912.
- O’Hare, Kate Richards. Americanism and Bolshevism. St. Louis, Mo.: F. P. O’Hare, 1919.
- O’Hare, Kate Richards. Socialism and the World War. St. Louis, Mo.: F. P. O’Hare, 1919.
- O’Hare, Kate Richards. In Prison. New York: A. A. Knopf, 1923.
- Engdahl, John Louis. Debs and O’Hare in Prison. Chicago: Socialist Party, [1919?].
- Foner, Philip S., and Sally M. Miller, eds. Kate Richards O'Hare: Selected Writings and Speeches. Baton Rouge: Louisiana State University Press, 1982.
- Miller, Sally M. From Prairie to Prison: The Life of Social Activist Kate Richards O'Hare. Columbia: University of Missouri Press, 1993.
- Zeuch, William Edward. The Truth About the O’Hare Case / by W.E. Zeuch. And Kate Richards O’Hare’s Address to the Court. St. Louis, Mo.: F.P. O’Hare, [1919?].