Due to her illustrations for Vrij Nederland, Fiep became known as a cartoonist, and from 1948 
onwards she was one of a group of illustrators and journalists involved in producing the satirical magazine Mandril, maandblad voor mensen. It was modelled on the American satirical magazine The New Yorker. Fiep did various illustrations for Mandril, including 
drawings to go with word games. Other illustrators who worked for this magazine were the brothers Charles and Wim Boost, Wim Bijmoer and Hugh Jans. This group of illustrators also organized exhibitions. For Fiep, the exhibition in the Stedelijk Museum of Amsterdam in 1956 was the most important of these; by then she had established a reputation as an illustrator and was asked to design the poster for the exhibition.