Jongejans was educated at the Haagse Acedemie van Beeldende kunsten from 1936 until 1940. He was trained to become a sculptor. However, he was more talented in drawing and photography and decided to go to the advertising department of the academy. He became the second man next to Paul Guermonprez at Co-op 2 design in Amsterdam in the 1940's.
From 1945 Jongemans works as an independent designer. He made designs for Ahrend, city of Amsterdam, for which he designed reports and calendars, Goed Wonen, Holland Festival, de Bijenkorf en het Koninklijk Verbond van Grafische Ondernemingen.
From 1948 until 1981 he taught letter knowledge and typography at the Instituut voor Kunstnijverheidsonderwijs in Amsterdam, which was later called the Gerrit Rietveld Academy. In response to changes in the practice of typographic design in the early fifties he started a separate section on typography at the Academy.