Brewster's sister and older brothers died in early childhood, inspiring Longfellow, a close neighbor, to write the poem The Open Window.

[2] Brewster's father gave him a gun and taught him to shoot, providing a means of collecting birds to study.

[4] In 1880, he became assistant in charge of the collection of birds and mammals in the Boston Society of Natural History, and in 1885 became curator of mammals and birds at the Museum of Comparative Zoology at Harvard University, working closely with director Alexander Agassiz and Elizabeth Hodges Clark,[5][6] where he served until his death, though after 1900 he cared for birds alone, and he left his position at the Boston Society of Natural History in 1887.

I did run and jump and try in every way to see if I could not reveal some lingering weakness but without doing so” [8] Brewster was a fellow of the American Association for the Advancement of Science, and in 1876 became president of the Nuttall Ornithological Club of Cambridge,[7] of which he was the founder in 1873.

[9] He was a co-founder, with Elliott Coues and Joel Asaph Allen, of the American Ornithologists' Union (AOU) in 1883[10] and served as its president from 1895 to 1898.

The group, with over half its officers being women, used its political power to have a Massachusetts law passed in 1897 outlawing trade in wild bird feathers and the 1900 Lacey Act, which prohibits interstate shipment of animals killed in violation of local laws.

He wrote: The Ernst Mayr Library of the Museum of Comparative Zoology at Harvard University maintains an archive of Brewster's journals, diaries, field notebooks, correspondence, and photographs.