Kazimierz Mordaszewski is a law graduate of the Nicolaus Copernicus University in Toruń, and also graduated from the Catholic University of Lublin.

He was active in the Independent Students' Union and the Union of Democratic Youth. He continued his activities in the democratic opposition in the People's Republic of Poland.

He gained professional experience as a trainee judge and then as a trainee legal advisor in the Legal Advisors Group, later working as an academic teacher at the Białystok branch of the Faculty of Law at Warsaw University. In 1989, he qualified as a legal advisor.

In 1990, he became involved in the process of creating new state security structures, taking the position of office director at the State Protection Office.

From 1997 to 2001, he served as Charge d'affaires at the Polish Embassy in the UAE, also being accredited to the State of Qatar. Upon his return to the country, in 2002, he opened a law practice within the partnership of KMMG Mordaszewski & Glinski attorneys and legal advisors.

In 2006, he joined the service of the Internal Security Agency, where he was Director of the Legal Bureau until 2012, and Deputy Head of the Agency from 2012 to 2015.

Kazimierz Mordaszewski was also a member of the Supervisory Board of PHZ Cenzin Sp. z o.o. and a member of the Board of the National Research and Development Center from 2010 to 2014.

He currently serves as a department director at the Ministry of State Assets.