After meeting with local Lions Club members, he borrowed $3,600 to take over the defunct Troy Call newspaper in 1948, becoming the nation's youngest editor-publisher, of the renamed Troy Tribune in Troy, Illinois, eventually building a chain of 14 weekly newspapers. His activism against gambling, prostitution, and government corruption while at the Troy Tribune influenced the newly elected Governor, Adlai Stevenson, to take a stand on these issues, creating national exposure for Simon that later resulted in his testifying before the Kefauver Commission.

In 1951, Simon left his newspaper and enlisted in the United States Army, during the Korean War. During his military career, Simon served as intelligence officer, and was honorably discharged in 1953, at the end of the war.

On April 21, 1960, Simon married his first wife, Jeanne Hurley Simon, a member of the state legislature. It was the first time in Illinois history that two sitting members of the Illinois General Assembly married. She was an integral part of her husband's rise to national prominence. She later became a successful Lawyer, author, and chair of National Commission on Libraries and Information Science. She died in February 2000 of brain cancer. Upon her death, Illinois Senator Dick Durbin delivered a tribute to Mrs. Simon on the senate floor. Their daughter, Sheila Simon, became the 46th Lieutenant Governor of Illinois in January, 2011. She previously served as a councilwoman in Carbondale, Illinois and was a law professor at Southern Illinois University.

In 1963, Simon was elected to the Illinois State Senate, serving until 1969 when he became the Lieutenant Governor of Illinois. As a Democrat, he served with Republican Governor, Richard B. Ogilvie. Their bipartisan teamwork produced the state's first income tax and paved the way for the state's 1969 constitutional convention, which created the fourth and current Illinois Constitution. The Ogilvie-Simon ticket was the only one in Illinois history in which the elected Governor and Lieutenant Governor were from different political parties.

Following his primary defeat for Governor in 1972, Simon founded the Public Affairs Reporting graduate program at Sangamon State University in Springfield, Illinois, which helped launch the careers of more than 500 journalists. Simon, who had written four books at the time, also taught a course entitled "Non-Fiction Magazine and Book Writing" at Sangamon State, and also taught at the John F. Kennedy School of Government in 1973.

Simon resumed his political career in 1974 when he was elected to Congress from Illinois's 24th congressional district, where he was re-elected four times. He was later redistricted to Illinois's 22nd congressional district

In 1978, Simon was the first recipient of the Foreign Language Advocacy Award, presented by the Northeast Conference on the Teaching of Foreign Languages in recognition of his Service on the President's Commission on Foreign Language and International Studies and his support for language study .

In 1984, he ran for, and was elected to the US Senate, defeating three-term incumbent Charles H. Percy in an upset election, winning just 50% of the vote.

Simon appeared on Saturday Night Live with host and singer Paul Simon on December 19, 1987.

Simon surged ahead in Iowa in October, and was, by December, the clear front-runner in that state. However, in February 1988, Simon narrowly lost the Iowa caucus to Representative Dick Gephardt of Missouri, and finished third in the New Hampshire primary the following week, with weak showings in Minnesota and South Dakota a week later. Out of money and momentum, Simon largely skipped the key Southern "Super Tuesday" primaries on March 8, concentrating on his home state a week later, where key local Democrats were running as Simon delegates on the delegate selection ballot, and wanted to attend the Democratic National Convention regardless of Simon's slim chance of winning the nomination. Simon won the Illinois primary, and decided to make a final effort in the Wisconsin Primary in early April, but dropped out after he finished behind Governor of Massachusetts Michael Dukakis, Reverend Jesse Jackson, and Tennessee Senator Albert Gore. Simon endorsed Dukakis, who won the Democratic nomination in July, with Jackson the last active challenger.

During the 1990s, Simon opposed both the Republicans' Contract with America, and President Bill Clinton's welfare reforms. He was one of 21 Senators to vote against the Personal Responsibility and Work Opportunity Act. In 1996, Simon joined thirteen other Senators (including his fellow U.S. Senator from Illinois, Carol Moseley Braun) in voting against the Defense of Marriage Act, which prohibited federal recognition of same-sex marriage.

Simon gained national prominence after criticizing President George H. W. Bush during the 1992 presidential campaign, after Bush claimed a central role in overseeing the collapse of the Eastern bloc of the Soviet Union. During a speech at Chicago's Taste of Polonia, Bush had aggressively promoted the success of his own presidency and his importance as Vice President in the Reagan administration's role in Eastern Europe. This was an attempt by Bush to carry Chicago's Polish community in order to win Illinois during the election. Bush's claims were roundly denounced by Simon, and Bush eventually lost the state in the general election, possibly due to Simon' remarks. Simon did not seek reelection in 1996.

Simon made a brief cameo appearance as himself in the 1993 political comedy film Dave.

Simon promoted a military response to Somalia during the presidency of George H.W. Bush. Simon was an outspoken critic of President Bill Clinton's response to the 1994 Rwandan Genocide. Simon believed that America should have acted faster, and Clinton later said his belated response was the biggest mistake of his presidency. He is, together with Jim Jeffords, supported by Canadian Lieutenant-General Roméo Dallaire, Force Commander of the United Nations Assistance Mission for Rwanda from 1993 to 1994, for actively lobbying the Clinton administration into mounting a humanitarian mission to Rwanda during the genocide. According to Dallaire's book Shake Hands with the Devil, he "owe[s] a great debt of gratitude" to both Senators.

Simon lived for many years in the small town of Makanda, Illinois, south of Carbondale, where he was a professor and Director of the SIU Public Policy Institute. While there, he tried to foster the Institute into becoming a think tank that could advance the lives of all people. Activities included going to Liberia and Croatia to monitor their elections, bringing major speakers to campus, denouncing the death penalty, trying to end the United States embargo against Cuba, fostering political courage among his students, promoting an amendment to the United States Constitution to end the electoral college, and attempting to limit the President to a single six-year term of office. During the electoral college fiasco that followed the 2000 election, Simon said: "I think if somebody gets the majority vote, they should be President. But, I don't think the system is going to be changed."

In May 2001, Simon remarried to Patricia Derge, the widow of former Southern Illinois University President David Derge.

Simon died in Springfield, Illinois following heart surgery at the age of 75 in 2003. WBBM-TV reported his death as a "massive gastric blow-out". Just four days before, despite being hospitalized and awaiting surgery, he had endorsed Howard Dean's 2004 presidential bid via a telephone conference call he conducted from his hospital bed. He was also an early supporter of Barack Obama's 2004 bid for Senate. After Simon's death, his daughter, Sheila, made a television commercial in which she declared "Barack Obama will be a U.S. Senator in the Paul Simon tradition." The ad was considered a major reason for Obama's surprise victory in the Democratic primary. In the Senate, Obama praised Simon as a "dear friend."

In July 2005, the Paul Simon Historical Museum was opened in Troy, Illinois, where Simon lived for 25 years. It included memorabilia from throughout his life, including the desk and camera from his days as a young Editor of the Troy Tribune, items from his presidential campaign, and his Lieutenant Governor license plates. The museum closed in June 2012, due to lack of funding. Paul Simon Chicago Job Corps is a government funding school in which was named after him. PSCJC is located in the city of Chicago in Little Village on South Kedzie Ave and is available to people between the ages of 16-24 who are looking to better themselves and create a positive Future for themselves.

Paul Simon was inducted as a Laureate of The Lincoln Academy of Illinois and awarded the Order of Lincoln (the State’s highest honor) by the Governor of Illinois in 1998 in the area of Government.