[1][2] In 1843 Day began practice in London, becoming a member of the Royal College of Physicians in 1844, and a fellow in 1847.
He was physician to the Western General Dispensary, and lecturer on materia medica at Middlesex Hospital.
In 1849 he became Chandos Professor of Anatomy and Medicine at the University of St Andrews, and obtained the M.D.
In 1863 changes were made in St Andrews by an act of parliament, and Day retired on a pension.
[1] Day was a contributor to periodical literature and the publications of learned societies.
[1] In 1841 Day married Ellen Anna, daughter of James Buckton, solicitor, of Doctors' Commons and Wrexham.