Capper was born in Garnett, Kansas. He attended the public schools and learned the art of printing. He became a newspaper publisher, eventually owning several newspapers and two radio stations. The best known of his publications, Capper's Weekly, had an enormous readership among farm families and served as the base of his political support in Kansas.

Capper first entered politics in 1912 when he became the Republican candidate for governor of Kansas. He was defeated by Democrat George Hartshorn Hodges. However, Capper was elected governor in the next election in 1914 and served as governor of Kansas from 1915 until 1919.

Capper did not run for reelection in 1918. Instead, he ran for election to the United States Senate and won. Capper became a long-serving senator, representing Kansas as one of its two senators for five 6-year terms. He was in the Senate from 1919 to 1949, and prominent among Republicans who supported the relief efforts and other policies of Franklin Delano Roosevelt's administration. He did not seek reelection in 1948.

Capper was particularly interested in issues relating to agriculture. Before his time as governor, he served as President of the board of regents of Kansas Agricultural College from 1910 to 1913. While in the Senate, he at times served as chairman of the Committee of Expenditures of the Department of Agriculture and the Committee on Agriculture and Forestry. He also at times served as chairman of the Committee on Claims and the Committee on the District of Columbia. He cosponsored the Capper-Volstead Act.

After retiring from the Senate, Capper returned to his home in Topeka, Kansas where he continued the newspaper publishing business until his death.