Bass was born on 30 January 1771 at Aswarby, a hamlet near Sleaford, Lincolnshire, the son of a tenant farmer, George Bass, and a local beauty named Sarah (née Newman). His father died in 1777 when Bass was 6. He had attended Boston Grammar School and later trained in medicine at the hospital in Boston, Lincolnshire. At the age of 18, he was accepted in London as a member of the Company of Surgeons, and in 1794 he joined the Royal Navy as a surgeon.