Daniel John Leech was born in Urmston, near Manchester, in January 1840. He began his formal studies as a student at Owens College in 1854 and became an apprentice of the medical practitioner, Mr Richmond of Manchester in 1856. The following year he enrolled in the Manchester Royal School of Medicine and subsequently qualified M.R.C.S. and L.S.A. in 1861. Leech spent several months in Paris pursuing post-graduate study before returning to Manchester in 1862 as a demonstrator of anatomy at the Manchester Medical School. After two years in this role he entered into partnership with his former teacher Mr Richmond, and remained in general practice for several years.
Leech continued to study during this time and received the degree of M.B. with first class honours in obstetrics from the London University in 1868 and was then appointed honorary physician to the Hulme Dispensary in 1870. In 1873 he joined the staff of the Manchester Royal Infirmary as honorary assistant physician, becoming physician in 1878, and embraced clinical teaching. He played a key role in the establishment of some of the city's other medical institutions, namely the Manchester and Salford Hospital for Skin Diseases and the Cancer Hospital and held honorary positions on the staff of both. He also served as consulting physician to the Deaf and Dumb Institute in Old Trafford.
In 1875 he had been made a member of the Royal College of Physicians, becoming a fellow in 1882, and in 1876 took his M.D. at the London University. That same year he accepted the co-lectureship of materia medica and therapeutics as Owens College and became professor in 1881. In this role Leech successfully endeavoured to form a materia medica museum to accompany the department. Leech played a prominent role in the administration of Owens College and later the Victoria University eventually becoming Pro-Vice-Chancellor of Victoria University. Leech also served on the General Medical Council, worked closely with the British Medical Council, in particular the local Lancashire and Cheshire Branch, and was a member of both the Manchester and Salford Sanitary Association and the Manchester Medical Society, serving as secretary to the former and president to the latter. He was also a member of the Pathological Society of London, the Royal Medical and Chirurgical Society, the Pharmaceutical Society of Great Britain, and the Manchester Literary and Philosophical Society. Leech gave the Croonian Lectures at the Royal College of Physicians in 1893 on the subject of 'The Pharmaceutical Action and Therapeutic Uses of the Nitrites and Allied Compounds'.
Leech died on 2 July 1900 after a long illness.