DeConcini served one elected term as Pima County Attorney (1973—1976), the chief prosecutor and civil attorney for the county and school districts within the county.

He was elected to the Senate in 1976 as a Democrat, having defeated Republican Party (GOP) U.S. Representative Sam Steiger for the open seat left by retiring GOP Senator Paul Fannin. Steiger had first won a bruising primary in 1976 against the more conservative U.S. Representative John B. Conlan.

Panama Canal

DeConcini sponsored an amendment (the DeConcini Reservation) to the Panama Canal Treaty of 1977 which allows the United States "to take such steps as each [the U.S. or Panama] deems necessary, in accordance with its constitutional processes, including the use of military force in the Republic of Panama, to reopen the Canal or restore the operations of the Canal, as the case may be."

Keating Five

DeConcini was widely noted as a member of the Keating Five in a banking and political contribution scandal during the 1980s which grew out of the U.S. Savings and Loan Crisis. The scandal involved Charles Keating and Lincoln Savings, contributing to DeConcini's retirement in 1994.

Senate committees

DeConcini served on the Senate Appropriations Committee where he chaired the Subcommittee on Treasury, Postal Service and General Government. He also served on the Subcommittees on Defense, Energy and Water Development and Foreign Operations. DeConcini also served on the Senate Judiciary Committee and chaired the Subcommittee on Patents, Copyrights and Trademarks, along with the Subcommittees on Antitrust, Monopolies and Business Rights, the Constitution and the Courts. DeConcini also served on the Select Intelligence Committee and was Chairman of the Committee in 1993 and 1994 and also chaired the Commission on Security and Cooperation in Europe (Helsinki Commission).

Appointments

In February 1995 DeConcini was appointed by President Bill Clinton to the Board of Directors of the Federal Home Loan Mortgage Corporation (Freddie Mac), where he served until May 1999.

In 2006 the former senator was selected by then Governor of Arizona Janet Napolitano (later Secretary of Homeland Security) to serve as a member of the Arizona Board of Regents.

DeConcini managed Shopping Centers, Inc. in Tucson, AZ from 1963—1964 and 1967-1973. He also managed family corporation and partnerships involved in Arizona and California real estate development projects. He has served on the boards of Global Health Science, Inc.,Schuff Steel Company, and Greater Arizona Savings Bank.