Margaret K. Knight, née Horsey, was born in Hertfordshire, England. She graduated from Cambridge University in 1926, and later earned a master's degree there.
From 1926 to 1936, she worked as a librarian, information officer, and editor for a journal published by the National Institute of Industrial Psychology. In 1936, she married Arthur Knight, a professor of psychology at the University of Aberdeen, Scotland, and joined the faculty as an assistant lecturer in psychology. In 1948 she was promoted to lecturer in psychology, a post she held until her retirement in 1970.
She and her husband co-wrote several textbooks, including A Modern Introduction to Psychology (1948), which went through numerous editions.
She was a well-known freethinker and advocate of scientific humanism, giving talks on BBC Radio. These lectures appeared in her book Morals Without Religion (1955). She compiled an anthology of humanist writings in 1961.