King was born in Fillmore, Utah Territory to Josephine and William King. He graduated from Brigham Young Academy in Provo, Utah and attended the University of Deseret in Salt Lake City. He served as a missionary of the Church of Jesus Christ of Latter-day Saints in Great Britain from 1880 to 1883. After holding local offices and serving two terms in the territorial legislature, he graduated from the law department of the University of Michigan at Ann Arbor. He later joined the Utah bar and practiced law. He held other territorial offices and was appointed as an associate justice of the Utah Supreme Court, serving between 1894 and 1896. After Utah became a state in 1896, King was elected to the United States House of Representatives and served in the 55th Congress from March 4, 1897 to March 3, 1899. He was not nominated for a second term, but when his replacement, B. H. Roberts, was denied his seat because he was a polygamist, King was elected to complete Roberts's term. He served from April 2, 1900 to March 3, 1901. He ran for the same position in 1900 and again in 1902, but lost both times. King was elected to the United States Senate four times, serving between March 4, 1917 and January 3, 1941. He was defeated for renomination in 1940. In 1918 and 1919, he served on the Overman Committee, which investigated seditious pro-German activity during World War I and Bolshevik-inspired anti-Americanism in the months following the war's end. He served as the President pro tempore of the Senate from 1940-41 during the 76th Congress. King remained in Washington, D.C., where he practiced law until April 1947. He returned to Utah and died there in 1949. He was buried at Salt Lake City Cemetery. King was married twice, first to Louisa Ann "Annie" Lyman, to whom he wed in 1889, and remained with her to her death on April 16, 1906. He was then married to Vera B. Sjodahl, a daughter of Janne M. Sjödahl, from 1912 to his own death in 1949. One of his sons by Vera, David S. King, served three terms in the U.S. House of Representatives and was a United States Ambassador to both the Malagasy Republic and Mauritius. One of his granddaughters, Jody Olsen, has served as Director of the Peace Corps since 2018. His paternal first cousin, Culbert Olson, was a Governor of California. King was a direct descendant of Edmund Rice, his family's English immigrant ancestor to Massachusetts Bay Colony, as follows:
