In 1899, after his father became ill, Carl Hayden dropped out of Stanford University, returning to Arizona to care for his family. Hayden soon became active in the Democratic Party and pursued a career in politics. Early in the new century, Hayden became active in the Democratic Party in Maricopa county, and won election to the Tempe Town Council. He built strong and productive relationships within the party, which later helped him in running for a higher political office. Agricultural interests in Tempe sent him to Washington to lobby for the funding of the Salt River Project--a role that both precipitated and augured Hayden's long interest in the politics of water. His political connections and stellar reputation paid off. Hayden was elected Maricopa County treasurer in 1904. He later claimed that those two years as treasurer provided him practical experience with public finance and budgets. It was also during this period (in 1903 to be precise) that Hayden enlisted in Arizona Territorial National Guard and was elected captain within two months of joining.
In 1906, Hayden ran for the position of Sheriff, at a time when Maricopa County was but a small agricultural settlement. The Sheriff's job carried a modest travel budget, and was considered a relatively lucrative position as the Sheriff's wages were based upon a percentage of fees collected. Hayden set upon the mundane tasks of maintaining order and transporting prisoners. He carried an unloaded revolver, which he later admitted was never used once during his time as Sheriff. Hayden spent a good portion of his time traveling throughout the region to meet other elected officials and built long-term relationships with Arizona’s regional leaders. Hayden built on these connections and when Arizona was admitted to the Union in 1912, he ran for the U.S. House of Representatives, becoming Arizona's first representative to Congress.
After serving in the House for almost 15 years, Hayden took a leap into the United States Senate, where he served from 1927 to 1969, making him the longest-serving member of Congress in history. During his time in Congress, Hayden was known by his colleagues to be a reclamation expert, given his expertise on water conservation.
He was stalwart defender of Arizona while the state was in the midst of a water rights dispute with California. The Supreme Court of the United States ultimately intervened and sided with Hayden in the case of Arizona v. California.
Overall, Hayden was known for his sense of fairness while serving in Congress. While Chairman of the powerful Senate Appropriations Committee, Hayden made sure members from both sides were treated fairly and given a voice on how and where to allocate federal funds. Hayden’s fair treatment of his colleagues not only earned him respect, but also maintained order and collegiality while he was serving as the President pro-tempore of the Senate. In the end, Hayden was a results-oriented politician who fostered amicable relationships wherever he went. Maricopa County’s farmland enjoyed order and civility while Hayden served as the county Sheriff, and the United States Senate was the most deliberative legislative body in the World while Hayden served as its President pro-tempore.