- May 2 US stops minting 20 cent coin
Edison's Phonograph
May 4 Thomas Edison's Phonograph shown for 1st time at Grand Opera House
Carbon Microphone
May 8 David Edward Hughes' paper on the idea for a microphone is read before the Royal Society of London by Thomas Henry Huxley
- May 13 Danvers State Hospital, a psychiatric hospital in Massachusetts, opens—later serves as inspiration for Arkham Sanitorium in the work of H.P. Lovecraft, which in turn inspired Arkham Asylum of the D.C. Batman universe
- May 14 Vaseline is granted a patent (U.S. Patent 127,568)
- May 19 Blanche Kelso Bruce appointed register of treasury by President Garfield
- May 21 4th Kentucky Derby: Jimmy Carter aboard Day Star wins in 2:37.25
- May 23 Attorney John Henry Smyth named US minister to Liberia
- May 24 CA Parker (Harvard) wins 1st American bike race at Beacon Park in Boston
H.M.S. Pinafore
May 25 W. S. Gilbert and Arthur Sullivan's comic opera "H.M.S. Pinafore" premieres in London, their first international success
- May 27 6th Preakness: C. Holloway aboard Duke of Magenta wins in 2:41.75
Spofforth's 10-20
May 27 Australian cricket fast bowler Fred Spofforth takes 10-20 (6-4 and 4-16) in quick-fire 9 wicket tour match win over the MCC in London
- May 31 German battleship SMS Grosser Kurfürst sinks, 284 killed
- May 31 US Congress accepts decrease in dollar circulation
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