William Brewster was born on July 5, 1851 in Wolfboro, New Hampshire to John and Rebecca (Parker Noyes) Brewster. He was the youngest of four children however, Brewster's sister and older brothers died in early childhood. Brewster attended Cambridge public schools, Washington Grammar School, and Cambridge High School. Around the age of 10, Brewster became close friends with Danile French, who's father was a hunter and amateur taxidermist. Brewster's father gave him a gun and taught him how to shoot which then allowed Brewster to start collecting birds. By 1865, Brewster had several cases of mounted birds along with a nest and egg collection. Over time his collection became one of the largest private collections in the US, which he later left to the Museum of Comparative Zoology of Harvard University. Brewster also collected detailed records of his observations and continued to do so for the rest of his life.
In 1880, Brewster became an assistant in charge of the bird and mammal collections at the Boston Society of Natural History. In 1885, he became curator of the bird and mammal collections at the Museum of Comparative Zoology of Harvard University, where he served until his death.
Brewster was a fellow of the American Association for the Advancement of Science, the President of the Nuttall Ornithological Club of Cambridge (1876), and co-founder and President (1895-1898) of the American Ornithologists' Union.