In some of his early articles, Gray adopted William Sharp Macleay's quinarian system for classifications of molluscs (1824), butterflies (1824), echinoderms (1825), reptiles (1825), and mammals (1825).

[3] During this period, he collaborated with Benjamin Waterhouse Hawkins, the noted natural history artist, in producing Gleanings from the Menagerie at Knowsley.

[5] During his 50 years employed at the British Museum, Gray wrote nearly 500 papers, including many descriptions of species new to science.

These had been presented to the museum by collectors from around the world, and included all branches of zoology, although Gray usually left the descriptions of new birds to his younger brother and colleague George.

He was an associate of entomologist Eliza Fanny Staveley, supporting her research and reading papers she had prepared to the Linnean and Zoological Societies of London.

He described more than 300 species and subspecies of reptiles, only surpassed by his successors at the British Museum, George A. Boulenger and Albert Günther and American zoologist Edward D.