Van Leijenhorst attended a Gymnasium in Amersfoort from May 1940 until June 1946, and applied at the Utrecht University in July 1948 majoring in Physics and Chemistry obtaining a Bachelor of Science degree in June 1950 and worked as a student researcher before graduating with a Master of Science degree in Chemistry in July 1954.

Van Leijenhorst became a Member of the House of Representatives after Willem Scholten was appointed as State Secretary for Finance in the Cabinet Biesheuvel I after the election of 1971, taking office on 3 August 1971 serving as a frontbencher chairing the special parliamentary committee for Cultural Minorities and spokesperson for Education, Social Work, Culture and deputy spokesperson for Media and Kingdom Relations.

After the election of 1986 Van Leijenhorst again returned as a Member of the House of Representatives, taking office on 3 June 1986.

In October 1993 Van Leijenhorst announced that he wouldn't stand for the election of 1994 and he continued to serve until the end of the parliamentary term on 17 May 1994.

Van Leijenhorst continued to comment on political affairs until his retirement in 1998, he died 3 years later at the age of 73.