Portman often looked to the past for inspiration. His painting of an old man in seventeenth-century costume demonstrates a fascination with the Dutch Golden Age. He focused on domestic subjects and portraits of fellow artists, such as the etcher Hendrik Voogd. His sketches of women's heads and studies of male figures show a dedication to anatomical precision. He favoured steady light and clear compositions over the emotional intensity of the Romantic movement. This approach resulted in paintings that feel balanced and observational.

Modern collectors appreciate the stillness found in Portman's work. His scenes of families by the water or a man reading at a table provide a sense of composure. These works offer a clear view of nineteenth-century life without the distraction of unnecessary ornament. The neutral colours and direct subjects allow his art to fit easily into various home styles. His work remains a document of Dutch social history and technical skill.