Kaczyński and his identical twin, Jarosław, were sons of Rajmund Kaczyński, a soldier who fought the German occupation of Poland, and his wife, Jadwiga, who taught Polish linguistics and served in a literary research institute. The brothers first attained prominence as child actors, appearing in Those Two Who Would Steal the Moon (1962). They were both educated at the University of Warsaw, and both later earned law degrees, Lech at the University of Gdańsk and Jarosław at Warsaw. During the 1970s, as students, they were active in anticommunist movements, and Lech was jailed briefly (1981–82) by the government.

Although both worked for a short time in education, by the early 1980s they had become active in Solidarity, the trade-union movement headed by Lech Wałęsa (Wałęsa, Lech). Lech Kaczyński held leadership positions in the movement, while his brother for a time edited its newspaper. When Solidarity came to power in 1989, Lech and Jarosław both began active careers in government. In 1990 they formed the Centre Agreement (Porozumienie Centrum), which Jarosław headed until 1998. Both brothers won election to the Sejm, the lower house of the Polish legislature, and they held a number of government appointments. By 1993, however, the pair had begun a falling out with Wałęsa, and in 2001 they cofounded the Law and Justice Party (Prawo i Sprawiedliwosc; PiS), headed (2001–03) by Lech and from 2003 by Jarosław.

The brothers did not fit neatly into traditional political categories. They were considered nationalist (even xenophobic by their critics) and aggressive in foreign policy, often hostile to the policies of the European Union, and sharply critical of the country's historical enemies, Germany and Russia. They took a strong stand against the perennial problem of corruption in Poland. At the same time, there were populist elements in their views; although they advocated a strong central government, they also promoted both tax cuts and a strong economic safety net. On social issues they were deeply conservative, strictly following the doctrines of the Roman Catholic Church.