Spare was born in London, United Kingdom, on December 30, 1886, into a working-class family. His father, Philip Newton Spare, came to London from Yorkshire in 1978 as he gained employment with the City of London Police. Austin Spare's mother, Eliza Osman, was born in Devon; she was the daughter of a Royal Marine.
Eliza Osman married Philip Newton Spare at St Bride's Church in Fleet Street in December 1879. The couple moved into a tenement called Bloomfield House on Bloomfield Place, King Street in Snow Hill. It was inhabited by the families of police officers, clerks, drivers, and market workers.
The Spare's first child to survive was Austin's brother John Newton Spare, born in 1882. Then his other brother, William Herbert Spare, was born in 1883 and Susan Ann Spare in 1885. Austin Spare also had a younger sister Ellen V. Spare, she was born in 1900.