In 1675 he drew the title plate of an illustrated work on mushrooms, the Theatrum fungorum oft Het toneel der campernoelien by Frans van Sterbeeck which was published in Antwerp in the same year by Jozef Jacobs.
Lenardi and van Westerhout collaborated for several years on other projects, including a large canvas entitled the Theological Allegory with the Assumption of the Virgin (Liverpool, Walker Art Gallery).
In the period from 1720 to 1725 the artist lived in Rome at Vicolo de Chiodarolli, in the Via delle Muratte, near the Trevi Fountain.
His stock was taken over by the Roman merchant Giacomo Billy, who had a shop near the Santa Maria in Vallicella church (the Chiesa Nuova).
[9] In the Vite Di Pittori (Lives of painters) written by Francesco Maria Niccolò Gabburri between 1730 and 1742 a nephew of Westerhout is mentioned.
On some of these he collaborated with another Flemish engraver active in Rome called Robert van Audenaerde, of whom he seems to have been the publisher.
[11] In addition to the Vetera Monimenta (1690–1698), van Westerhout worked on other publication projects, including the following: Venanzio Monaldini published in 1765 in Rome a series of eleven plates (plus a title plate) under the title Raccolta di Diverse Caricature delineate ed incise da Arnoldo van Westerhout Obra per la prima polta date in luce.
These plates were allegedly designed and engraved by Arnold van Westerhout and sold from his bottega in Rome in 1687.
The Italian art historian Alberto Milano is of the view that this series of plates has nothing to do with the original one published by van Westerhout in Rome in 1687.
[23] Even though van Westerhout was admitted as a master of the Guild of St. Luke of Antwerp as a painter, only very few of his canvases have been identified to date.
[24] In addition there is the Theological Allegory with the Assumption of the Virgin (Liverpool, Walker Art Gallery) which he painted in collaboration with Giovanni Battista Lenardi.