At the beginning of the seventeenth century he arrived at the court of King Władysław IV, where he quickly gained considerable influence, and most notably was appointed the "starost" of Sochaczew. In 1640, for the first time, he became a senator in the Sejm (parliament) despite the scandal (before the meeting of the parliament he was accused of rape). In 1645 he was appointed the Speaker of the Chamber of Deputies. Following this, in the name of the king, he was involved in the secret negotiations with the Cossacks that would support Poland in the Turkish expedition, however in 1646, when the secret case came out to the public, the nobles and senators of the Sejm, opposed the campaign. At the same time Radziejowski gained the influence at the court of the queen, appearing on the same parliament to defend her royal interests. In 1648 he participated in the Battle of Piławce. Radziejowski was one of the accused noblemen of contributing to the escape of the Polish army during that battle. He then campaigned for the election of John II Casimir and supported his marriage to the widow of Władysław IV, Queen Marie Louise Gonzaga. In 1648 he was the elector of John II Casimir representing the city of Łomża. In May 1650, he married (his two previous wives died) the newly widowed Elżbieta Służka, an heir to a great fortune after her deceased husband, great magnate Adam Kazanowski. In the same year he was appointed the Deputy Chancellor of the Crown, which was accompanied by another scandal, because during his speech in the parliament, while receiving the royal seal, the Grand Marshal of the Crown Prince Jerzy Sebastian Lubomirski turbulently stated that Radziejowski bought his position and post in the government.