Carl Frederick Geiser was born in Orrville, Ohio on December 10, 1910. He was the oldest of six children; his father, a farmer, died in the influenza epidemic at the end of World War I, and his mother a year later of tuberculosis. His maternal grandparents, Swiss immigrants who spoke little English, raised Geiser and his siblings. The young Geiser received his primary education in a one-room schoolhouse while helping to tend the family's sixteen-acre farm. Upon his graduation from Orrville High School in 1928, he enrolled in the YMCA School of Technology (later Fenn College) in Cleveland, where he majored in electrical engineering.

In 1932, following the establishment of diplomatic relations between the United States and the Soviet Union, Geiser was part of the first National Student Federation mission to travel to the newly recognized country. This visit had a decisive influence on shaping Geiser's political thinking. Impressed by the Soviet system and the tenets of socialist ideology, Geiser joined the Young Communist League upon his return to Ohio. He became an active force in the American Student Union in Cleveland and served as a delegate to the First Student Congress Against War and Fascism held in Chicago. It was there that Geiser met his future wife Sylvia, a teacher and organizer who shared his political fervor. The couple moved to New York where they were absorbed into a dynamic culture of political activism and organizing. Geiser wrote press releases and edited International Labor Defense bulletins, organized for the League against War and Fascism, and in 1936 was elected to the National Committee of the Young Communist League.

On April 13, 1937 Geiser boarded the S.S. Georgicto join the International Brigades massing in defense of the Spanish Republic. He served as an ammunition carrier at the Battle of Brunete, saw action at Quinto, and advanced to the rank of Lieutenant. Following the Battle of Belchite in September 1937, Geiser was promoted to Political Commissar and charged with the organization of a training school for commissars at Tarazona. Wounded at the conflict at Fuentes de Ebro, Geiser was hospitalized for three months. Returned to the front as Commissar of the Mackenzie-Papineau Battalion in January 1938, he was captured by fascist forces on April 1, 1938. For the next year, he was interned at San Pedro de Cardeña, along with over 650 International Brigades prisoners. Through the efforts of the Friends of the Abraham Lincoln Brigade and the U.S. State Department, Geiser and a group of 71 Americans were released in April 1939.

Geiser returned to New York and secured an engineering position with Liquidometer, a manufacturer of aeronautic equipment. Working with the company in various capacities for the next 40 years, Geiser filed numerous patents and, as a research director, supervised the testing of a component used in the first lunar mission. He also served briefly as president of Local 1227 of the United Electrical Radio and Machine Workers of America. He and Sylvia had two boys, Jim and Pete, before divorcing in 1946. With his second wife Doris he had a son and a daughter, David and Linda. In 1956 Geiser enrolled at Columbia University's School of General Studies as a psychology major, and graduated with a B.S. degree cum laude in 1963.

By the early 1970s, Geiser turned his attention once more to Spain. At the promptings of his wife, Geiser enrolled in a memoir-writing class. The essay he wrote on a Christmas concert held in San Pedro de Cardeña found publication in The New York Times, and its positive reception provided the impetus for Geiser to produce a more extensive treatment of his concentration camp experience. Upon retirement at age 71, Geiser began to write a comprehensive history of American volunteers captured during the Spanish Civil War. With the assistance of fellow prisoner Robert Steck, Geiser amassed biographical information on the 120 Americans incarcerated in Spanish prisons. He also corresponded with over 150 veterans worldwide to solicit their reminiscences, and traveled to archives in the United States and Europe to conduct research. Ring Lardner, Jr., (whose brother James was killed in action while fighting with the International Brigades) and members of Veterans of the Abraham Lincoln Brigade, eager to see the project to fruition, provided financial support. Five years of research and writing culminated in the production of a 900-page manuscript. Prisoners of the Good Fight, a shortened version of his account, was published in 1986.