He graduated from Lamont High School in 1947 and enlisted in the United States Army, serving in Europe during the Korean War.

[1] This caught the attention of attorney John Culver of Cedar Rapids, Iowa, who enlisted Clark to help run his congressional campaign in 1964.

After their victory, Clark became Culver's administrative assistant, and the pair modernized the Iowa Democratic Party's grassroots efforts in the state, building up a sophisticated voter turnout organization that progressed from names on index cards to computerized databases.

But in early 1972, Culver decided that defeating entrenched incumbent Republican Senator Jack Miller was impossible and bowed out of the race.

[3] He served on the United States Senate Committee on Foreign Relations and chaired the Subcommittee on Africa, developing considerable expertise on the Angolan Civil War.

In 1976, he authored the Clark Amendment, which barred aid from the U.S. government to private groups engaged in military or paramilitary operations in Angola.

Clark joined the Aspen Institute and in 1983 founded its Congressional Program, which sought to educate members of Congress on foreign affairs issues.