Having attended Toastmasters to improve his initially abysmal public speaking style, Evans served in the Washington State House of Representatives from 1956 to 1965 before being elected governor.Despite being a Republican and a self-styled conservative, Evans became known for his administration's liberal policies on environmental protection (he founded the country's first state-level Department of Ecology, which became Nixon's blueprint for the federal EPA) and strong support of the state's higher education system, including founding Washington's system of community colleges. He fought unsuccessfully for a state income tax.

Evans served as governor from 1965 through 1977, still the only governor to serve three four-year consecutive terms and the second to be elected to three terms following Arthur B. Langlie in Washington state history. A 1981 University of Michigan study named him one of the ten outstanding American governors of the 20th century. He declined to run for a fourth term. Serial killer Ted Bundy served as a campaign aide for Evans and maintained a close relationship with the Governor. During the 1972 campaign, Bundy followed Evans' Democratic opponent around the state, tape recording his speeches and reported back to Evans personally. A minor scandal later followed when the Democrats found out about Bundy, who had been posing as a college student.^{[citation needed]}

From 1977 to 1983 Evans served as the second president of The Evergreen State College in Olympia, which Evans had created in 1967 by signing a legislative act authorizing the formation of the college. The largest building on the Evergreen campus is named the Daniel J. Evans Library in his honor. In 1983, Governor John Spellman appointed Evans to the United States Senate to fill a seat left vacant by the death of longtime senator Henry M. "Scoop" Jackson. Evans won a special election later that year against Mike Lowry and filled the remainder of Jackson's unexpired term, retiring from politics after the 1988 elections. He was not happy as a U.S. Senator; he wrote an April 1988 piece in The New York Times Magazine, "Why I'm Quitting the Senate", in which he complained of "bickering and protracted paralysis".