Kanjorski is a lifelong resident of Nanticoke, just outside Wilkes-Barre. He attended Temple University and the Dickinson School of Law. He also served in the United States Army. Before his election to Congress, Kanjorski was a trial attorney in Northeastern Pennsylvania. During that time, he served as a worker’s compensation administrative law judge and served as solicitor to several communities.

Kanjorski ran four times for Congress in the early 1980s. He ran in a special election to replace longtime Congressman Dan Flood in 1980. He ran as an independent and finished a distant fourth behind State Representative Ray Musto. He ran against Musto in the Democratic primary election later that year and finished third. Musto lost to Republican James Nelligan in November. In 1982, he finished second in the primary election to college professor Frank Harrison, who went on to unseat Nelligan. On the fourth try, in 1984, he sought a rematch in the Democratic primary against Harrison and won. Despite the wide perception that 1984 was a disaster for Democrats (largely due to the landslide reelection of Ronald Reagan), Kanjorski won by a decisive 17-point margin. Kanjorski's opponent in 1986 was a younger, well-financed candidate, Marc Holtzman. The race was initially seen as one of the hottest in the country. However, Kanjorski won by 41 points, still his largest margin of victory in a contested election.

He was unopposed for reelection in 1988 and 1990 and didn't face another credible opponent until 2002. That year, Hazleton mayor Lou Barletta faced him and briefly gained momentum when reports that Kanjorski improperly used his influence to funnel millions of dollars in federal funds to companies owned by family triggered an FBI investigation. However, Kanjorski got an unwitting assist from the state legislature. In the neighboring 10th District, longtime congressman Joseph McDade's successor, Don Sherwood, won by only 515 votes in his initial 1998 bid and faced another squeaker in 2000. The legislature moved heavily Democratic Scranton, the heart of the 10th for more than a century, into the 11th. If anything, this made the heavily Democratic 11th even more Democratic. There was nothing altruistic about this remap, as the Republican-controlled legislature's goal was to make Sherwood safer while packing most of the Democratic votes in northeast Pennsylvania into the 11th. Kanjorski was reelected by 13 points and was unopposed for reelection in 2004.

As a Congressman, Kanjorkski has had a mixed voting record - liberal on economics and moderate on social issues. He has served on the Financial Services Committee since his first election in 1984 and is the second-ranking Democrat on that committee. Kanjorski usually plays behind-the-scenes roles in the advocacy or defeat of legislation and steers appropriations money toward improving the infrastructure and economic needs of Northeastern Pennsylvania.