His most notable creation was Miffy (Nijntje in the original Dutch), a small rabbit drawn with heavy graphic lines, simple shapes and primary colours.

Bruna also created stories for characters such as Boris Bear, Poppy Pig, Snuffy, Lottie, Farmer John, Kitty Nelly, and Hettie Hedgehog.

[4] Back in the Netherlands he attended the Rijksakademie van beeldende kunsten in Amsterdam but soon quit; he said afterwards that he had no talent as a painter and could not draw perspective.

His most recognized illustrations were for the Zwarte Beertjes (English: little black bears) series of books, including The Saint, James Bond, Simenon, and Shakespeare.

Bruna has also been noted to have been influenced by the Dutch graphic design movement, De Stijl, in particular the work of architect Gerrit Rietveld.