In 1878, Mellery was commissioned by Charles de Coster to illustrate an article on Holland, and to this end he journeyed to the island of Marken (off the Dutch coast), then a simple, traditional fishing community. His experiences there not only reinforced his tendency towards introspection but also helped define his lasting preference for a sober and quiet realism that rapidly infected the mood of his paintings and drawings. Or, as he himself put it: 'He who will manage to have us forget colour and form at the price of emotion will achieve the highest goal of all.'

It was in his drawings above all that this realism attained a rare degree of poetry, revealing l'âme des choses (the soul of things), the invisible behind the visible in the familiar, day-to-day world of his house and garden. The elements of silence and secrecy, together with his internalised perception of the world had a crucial influence on Fernand Khnopff.

In a less introverted, more public domain, Mellery sought to create an allegorical art in which classically-inspired figures, set against flat, golden backdrops and frequently accompanied by texts, expanded universal ideas or truths. Such works, greatly informed by his visit to Italy in 1871 and a subsequent trip to Germany, Switzerland and Austria in 1887, were generally conceived on a monumental scale as programmes for major public decorative cycles. While Mellery never realised this ambition, these works were exhibited in reduced version, both at Sâr Péladan's Salon de la Rose+Croix in Paris in 1897, and in Brussels.

Mellery exhibited regularly at Les XX as an invited artist from 1885 (1888, 1890, 1892), and he subsequently contributed four times to La Libre Esthétique (1894, 1895, 1899, 1908). He was a founding member of Pour l'Art, and a member of Kunst van Heden, the Société Royale Belge des Aquarellistes and the Belgian Académie Royale. Mellery also produced book illustrations, such as those for Lemonnier's La Belgique (1888), and executed designs for sculpture, including the small bronze statuettes personifying various trades for the park of Le Petit Sablon in Brussels in 1882, and a few years later, symbolic figures on the façade of the Palais des Beaux-Arts, now the Musée d'Art Ancien in Brussels.