In 1994, he was elected to the office of the Ohio State Senate, where he served for two years. After this, he was elected to the U.S. House of Representatives in the 10th District of Ohio in 1996. He has served in this position until the present time, through five consecutive terms. His district includes his home town of Cleveland, as well as several other districts.

His work as a Representative has been active. He helped introduce the United States National Health Insurance Act of 2003. He has taken exception to the Patriot Act, the flag-burning amendment, and has spoken out against improper voting machine design regarding Diebold Election Systems, particularly regarding the alleged fraud in which President Bush was supposed to have rigged the voting.

Dennis Kucinich has also been outspoken in criticizing the Iraq war, he has called for nuclear disarmament, and he has said that the United States should withdraw from the NAFTA treaty. He has been fiery on U.S. foreign and war policy in general; for example, he explained his vote against the symbolic "9/11 Commemoration" by stating that it left out "the lies that took us into Iraq, the lies that keep us there, the lies that are being used to set the stage for war against Iran and the lies that have undermined our basic civil liberties here at home."

He made a run for President in 2004. Although he won ringing endorsements from many Democratic and Green party members, including Ralph Nader, he eventually lost the Democratic race to the more popular Dean and Kerry. As of December 2006, he has announced his seeking of the office of President for the 2008 race, doing so from his home town of Cleveland.

Dennis Kucinich is mostly classifiable as a medium-left Democrat. However, he does not necessarily let this stop him from voting across party lines when the issue at hand is something that he has a firm opinion about. He has changed his stance from anti-choice to pro-choice regarding a woman's right to an abortion. He did meekly go along with the war drum on Iraq in the early days of the Bush administration, before backing up and out of the support for the war and has since loudly protested the situation in the Middle East. He has even gone so far as to make public visits to Middle Eastern countries, making speeches and appearances in which he denounced what the United States is doing over there.