Hadley served as a senior foreign and defense policy advisor to then-Governor Bush during the 2000 presidential campaign and worked in the Bush-Cheney Transition on the National Security Council. Previous to this position, he was a partner in the Washington, D.C. law firm of Shea & Gardner and a principal in The Scowcroft Group, Inc., an international consulting firm. Hadley was what in Rise of the Vulcans: The History of Bush's War Cabinet, James Mann called, "A Pentagon aide to Wolfowitz in the George H.W. Bush administration," serving as the Assistant Secretary of Defense for International Security Policy from 1989–1993. In that position, he had responsibility for defense policy toward NATO and Western Europe, on nuclear weapons and ballistic missile defense, and arms control. He also participated in policy issues involving export control and the use of space. Hadley served as Secretary of Defense Cheney's representative in talks led by Secretary of State James Baker that resulted in the START I and START II Treaties.

Hadley previously served in a variety of other capacities in the defense and national security field, including serving from 1986–1987 as Counsel to the Special Review Board established by President Ronald Reagan to inquire into U.S. arms sales to Iran, as a member of the National Security Council staff under President Gerald Ford from 1974–1977, and as an analyst for the Comptroller of the Department of Defense from 1972–1974. Hadley has been a member of the Department of Defense Policy Board, the National Security Advisory Panel to the Director of Central Intelligence, and the Board of Trustees of Analytical Services, Inc. ("ANSER").

His professional legal practice focused on business problems of U.S. and foreign corporations particularly as they involve international business, regulatory, and strategy issues. He received a B.A. degree in government from Cornell University in 1969, where he was a member of Phi Kappa Psi Fraternity, the Cornell University Glee Club, and the Quill and Dagger society. He later received a Juris Doctor (J.D.) degree from Yale Law School and served as an officer in the United States Navy from 1972 to 1975.

In January 2001, as George W. Bush prepared to take office, Hadley served on a panel for nuclear weapons issues sponsored by the National Institute for Public Policy, a conservative think tank. Other members of the panel included Stephen Cambone, William Schneider, and Robert Joseph. This panel advocated using tactical nuclear weapons as a standard part of the United States defense arsenal. In 2002, Hadley was a member of the White House Iraq Group. He admitted fault in allowing a disputed claim about Iraq's quest for nuclear weapons material to be included in Bush's January 28, 2003 State of the Union Address.

On July 22, 2003, Hadley offered his resignation to Bush because he had "failed in that responsibility" and that "the high standards the president set were not met." Bush denied Hadley's request. Amid this, The Times of London reported that Hadley was Bob Woodward's source for Valerie Plame's name in the CIA leak scandal, but this report proved to be false when Richard Armitage admitted that he was Woodward's source.

In former president Jimmy Carter's book, Hadley is referred to without being named as personally denying Carter permission to visit Syrian president Bashar al-Assad in early 2005 due to "differences with Syria concerning U.S. policy in Iraq." Hadley is married to Ann, a Justice Department lawyer, and has two adult daughters.

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