Ernest Reynolds studied medicine at Owens College and qualified in 1883. He took the London MB in 1884 and the MD in 1885. He originally worked at various mental hospitals in the region, and was appointed to the MRI honorary staff in 1899. Reynolds is perhaps best remembered for discovering the presence of arsenic in beer in 1900, following an outbreak of herpes zoster associated with beer drinkers. Arsenic was traced to impurities in sulphuric acid used in the production of sugar for beer. This discovery led to improvements in the purity of beer and won an international reputation for Reynolds.
Despite this discovery, Reynold's main interest was neurology. He was appointed professor of clinical medicine at the University of Manchester in 1912. He was attached to the 2nd Western General Hospital during the First World War. He was president of the Manchester Medical Society in 1912. Reynolds was secretary of the Section of Psychological Medicine at the 1902 meeting of the BMA in Manchester and was vice-president of the section of neurology and psychological medicine at the 1912 BMA meeting in Liverpool.