He worked with vertebrates, primarily fishes. From 1864 he was curator at the Zoological Museum in Oslo. In 1882 he became its director and from 1884 was a professor there.
He described many new species of fish, spiders and other organisms.
A popular book Norges pattedyr written by him was on the mammals of Norway. This book was to influence Charles Elton to study the periodic fluctuations in animal populations.
The work by Elton on animal fluctuations, particularly those of lemmings challenged the idea that there was a balance in nature. Robert Collett was the oldest child of Peter Jonas and Camilla Collett.
He had three younger brothers.