He returned to Flanders where he worked in Brussels as a portrait painter to the Governors of the Habsburg Netherlands Archduke Albert and the Infanta Isabella.
[5] He must have achieved success early on as he painted a portrait of the local doge Agostino Doria sitting on a throne with his family not long after his arrival.
[7] On 20 May 1614, 10 days after the miniatures were delivered, van Deynum formally entered the service of the Archdukes and was named an illuminator of the Brussels court.
[7] He collaborated with Servaes de Coulx and Jacob van der Laemen on the Adoration of the Magi (Enghien, convent of the Capuchins).
The signed and dated Portrait of a Genovese lady in a black dress (1610, Palazzo Bianco, Genoa) plays an important role in identifying his oeuvre.
[8] The Portrait of a Genovese lady in a black dress of 1610 shows his work to be strongly linked to the forms of expression of the end of the 16th century and to closely follow the cold and impersonal approach of Frans Pourbus the Elder.
[8] Apart from three female portraits and one male portrait given to the artist, the best known works of the artist are a miniature representing The doge Agostino Doria sitting on a throne with his family (private collection) and a painting depicting Archduke of Austria and Isabella of Austria at a reception ball in Genoa (private collection).
The painting shows in the foreground the doge, his wife and their four children sitting on a large terrace next to a classicizing building, and offering in the back a view over the Ligurian coastline.
The painting consists of two planes: on the foreground is shown a lively gathering of elegantly dressed people in distinctive poses while in the back there is a wide landscape occupying three quarters of the composition.
[12] It is possible that Guilliam was responsible for the portraits, which are very delicately rendered with a lot of detail while the collar and other parts which do not display the same lightness of touch were executed by Antoni.