Frenzel was elected as a Republican to the 92nd, 93rd, 94th, 95th, 96th, 97th, 98th, 99th, 100th, and 101st congresses, serving from January 3, 1971 to January 3, 1991, and was the ranking Republican on the House Budget Committee and a member of the influential Ways and Means Committee. He was a Congressional Representative to the General Agreement on Tariffs and Trade (GATT) in Geneva for 15 years. Frenzel became known as an expert in budget and fiscal policy, election law, trade, taxes and congressional procedures, and was a negotiator in the 1990 budget summit. During the Iran–Iraq War of the 1980s, Frenzel was a proponent of economic ties to the regime of Saddam Hussein, and opposed congressional efforts to condemn Iraqi war crimes such as the infamous Halabja chemical attack, the deadliest chemical-weapons attack in history, on the grounds that they would disrupt future trade with Iraq. He also served as vice chairman of the Committee on House Administration, and vice chairman of the Commission on Congressional Mailing Standards. He did not run for re-election to the House in 1990.