Hamlin was born in Brevard, North Carolina. In 1869 moved to Missouri with his parents, who settled in Leasburg, Crawford County. He attended the common schools and Salem Academy, where he studied law. He was admitted to the bar in 1882 and commenced practice in Bolivar, Missouri.

Hamlin was elected as a Democrat to the Fifty-eighth Congress (March 4, 1903 – March 3, 1905). He was an unsuccessful candidate for reelection in 1904 to the Fifty-ninth Congress. Hamlin was elected to the Sixtieth and to the five succeeding Congresses (March 4, 1907 – March 3, 1919). He served as chairman of the Committee on Expenditures in the Department of State (Sixty-second through Sixty-fifth Congresses). He was an unsuccessful candidate for renomination in 1918.

He resumed the practice of law in Springfield, Missouri, until November 1935, when he retired and moved to Santa Monica, California, where he died February 16, 1950. He was interred in East Lawn Cemetery in Springfield, Missouri.

• United States Congress. "Courtney W. Hamlin (id: H000119)". Biographical Directory of the United States Congress.

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