Clark began his government service in 1906, when he was appointed assistant solicitor to the state department. During World War I, Clark worked in the Attorney General's office. He also participated in creating the regulations for the Selective Service.

In 1928, as Under Secretary of State to President Calvin Coolidge, Clark wrote the "Clark Memorandum on the Monroe Doctrine", which repudiated the idea that the United States could arbitrarily use military force in Latin America.

Clark served as U.S. ambassador to Mexico from 1930—1933. Regarding his service, President Hoover said, "Never have our relations been lifted to such a high point of confidence and cooperation."

In April 1933, Clark was called to serve in the LDS Church as the Second Counselor in the First Presidency to President Heber J. Grant. He replaced Charles W. Nibley, who had died in December 1931. This call was unusual, not only for the delay between Nibley's death and Clark's call, but also because counselors were generally selected from within the general authorities of the church. (Clark had also never been a stake president or bishop in the church. He wasn't even very active in the church at that point, due to his duties as ambassador.) In September 1934, Grant's First Counselor Anthony W. Ivins died. In October 1934, Clark was ordained an apostle and member of the Quorum of the Twelve Apostles for purposes of seniority. Immediately thereafter, he was set apart as Grant's First Counselor, with David O. McKay as the Second Counselor.

After Grant's death, Clark and McKay were also First and Second Counselors, respectively, to George Albert Smith. However, when Smith died and McKay became President of the Church, he surprised some by choosing Clark as his Second Counselor, with Stephen L. Richards as First Counselor, citing Richards' longer tenure as an apostle as his only reason for doing so. It was after this that Clark famously remarked that "In the service of the Lord, it is not where you serve but how. In the Church of Jesus Christ of Latter-day Saints, one takes the place to which one is duly called, which place one neither seeks nor declines." Clark was returned to the position of First Counselor after Richards' death in 1959 and continued to serve in that capacity until his own death on October 6, 1961.

• Brigham Young University/Selected Clark references
• Grampa Bill's General Authority Pages
• Biography on the J. Reuben Clark Law Society website

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