Holtrop was trained as an artist from 1932-1935 at the Academy of Visual Arts in Rotterdam. In 1939, he studied at the Académie de la Grande Chaumière in Paris, and in 1941 at the Academie Kunstoefening Arnhem.

He was a pupil of Herman Mees, David Bautz, Antoon Derkzen van Angeren and Gijs Jacobs van den Hof, among others. Holtrop became proficient in a wide range of visual arts. He was a sculptor, painter, draughtsman, graphic and monumental artist.

He himself was the teacher of Wim Bos, Bas Kleingeld, Wim van Oostrom, Annemiek Punt and Jozef de Bot. Together with artists such as Ben Akkerman, Jan Broeze, Johan Haanstra and father and son Folkert Haanstra, he founded De Nieuwe Groep in 1945.

In addition to his work as an artist, Holtrop also published about the visual arts. His work was exhibited several times in Dutch museums.

The Stedelijk Museum in Amsterdam and the Haags Gemeentemuseum, among others, have works by Holtrop in their collections. His sculpture "the Scream" has been placed as a war monument on the Ressingplein in Delden.