Hewitson was born in Newcastle. He was of delicate health, but a large legacy enabled him to devote himself to scientific studies and his book British Oology was published in Newcastle between 1833-42.
After 1848 he lived at Oatlands Park in Surrey, and turned his attention to butterflies. He bought specimens from travellers and naturalists, whose expenses he often paid, and formed probably the most complete collection of diurnal lepidoptera in the world. These he left to the Natural History Museum. Hewitson displays in his book a considerable talent as an artist and as a pictorial describer of butterflies, he stands unequalled. Hewitson left his library of works on natural history and a legacy of £3000 to the Natural History Society of Newcastle.