Gladys Mitchell was born in the village of Cowley, near Oxford, in April 1901; she is of Scottish descent on her father's side. Her early years were spent in Oxfordshire and Hampshire, but in 1909 her family moved to Middlesex, where she was educated, at the Rothschild School, Brentford, and the Green School, Isleworth. She went on to Goldsmiths' College and University College, London, qualifying as a teacher and gaining an extra-mural diploma in European history.
She became a teacher of English, history and games, and though she found success as a writer, she remained in the teaching profession until her first retirement in 1950. Her first post was in a small Church of England school, St. Paul's, Brentford, where she stayed for four and a half years. She then taught at St. Ann's Senior Girls' School in Hanwell, remaining until the outbreak of war in 1939, specializing in history and athletics, and coaching, among others, a county hurdles champion in the mid-30s. After a year's enforced absence from teaching owing to illness, she joined the staff of Brentford Senior Girls' School where, in addition to her usual history and games, she taught elementary Spanish, and where she remained until she retired in 1950.
After nearly three years of retirement, Miss Mitchell was invited to the Matthew Arnold County Secondary School for Girls to judge an inter-House gymnastics competition and to address the school. At the conclusion of her speech, the headmistress invited Miss Mitchell to join her staff the following term, and although she had no intention of returning to teaching, the omens seemed favorable and she accepted the post offered her. In addition to teaching history and English, she wrote a number of plays for the girls to perform, including versions of the Greek legends of Theseus and Jason, the story of Jonah, and the Norse legends; an adaptation of "The Frogs" by Aristophanes; and a musical called "Alice Again," based on the Lewis Carroll classics. Miss Mitchell finally retired from teaching in 1961, at the age of 60.
During her teaching career, Gladys Mitchell lived first in Brentford and then in Ealing, but on her retirement she moved to the country, to Corfe Mullen in Dorset, where she was able to pursue two of her principal interests, the investigation of pre-historic sites and the study of mediaeval architecture. She had long been an enthusiastic student of Freud; and she attributed her interest in witchcraft to the influence of her friend Helen Simpson. Miss Mitchell received membership in the British Olympic Association, a testament to her enduring interest in athletics.
Miss Mitchell wrote her first novel in 1923, but it was rejected, as were three others: in desperation, she tried her hand at a detective story, and the result was Speedy Death, which Victor Gollancz agreed to publish despite the fact that it "had every fault under the sun." That book's detective, Mrs. Bradley, would be featured in sixty-five more novels and several short stories, until her creator's death in 1983.