- May 1 British liner Lusitania leaves NY for Liverpool
- May 1 German submarine torpedoes US tanker Gulflight
- May 2 Old Fordham Road in Bronx renamed Landing Road
Event of Interest
May 3 John McCrae writes the poem "In Flanders Fields"
John McCrae
- May 6 The Allies on Cape Helles launch three attacks to enlarge their beachheads; after terrible losses, they advance about three miles
- May 7 RMS Lusitania sunk by German submarine off the southern coast of Ireland; 1198 lives lost
- May 8 41st Kentucky Derby: Joe Notter aboard Regret wins in 2:05.4
- May 9 German & French fight Battle of Artois
- May 10 Zeppelin drops hundred of bombs on Southend-on-Sea, Essex, England
Historic Invention
May 10 Canadian physician Cluny MacPherson first presents his gas mask invention to the British War Office
Cluny MacPherson
- May 12 Croatians plunder Armenia, killing 250
- May 12 Franklin K Mathiews presents idea of "Book Week"
- May 13 US Secretary of State Bryan sends a note to Germany demanding that Germany disavow the attacks on the Lusitania and make immediate reparations; however, the note is written only to 'pacify exited public opinion', according to Bryan
- May 17 40th Preakness: Douglas Hoffman aboard Rhine Maiden wins in 1:58
- May 17 Cubs George "Zip" Zabel relieves with 2 outs in 1st & winds up with 4-3 19-inning win over Brooklyn in longest relief job ever
Event of Interest
May 17 Last liberal British government of H. H. Asquith falls
H. H. Asquith
- May 17 National Baptist Convention chartered
- May 20 Bataafsche Petroleum Me begins oil extraction of Maracaibo
- May 22 Local train collides with troop train killing 226 in Gretna, Scotland
- May 22 Lassen Peak erupts with a powerful force, only mountain other than Mount St. Helens, to erupt in the continental US during the 20th century
- May 23 Italy declares war on Austria-Hungary during WW I
Historic Invention
May 24 Thomas Edison invents telescribe to record telephone conversations
Thomas Edison
- May 25 Second Battle of Ypres ends with 105,000 casualties
- May 25 Chicago Claude Hendrix no-hits Pitts (Federal League), 10-0
- May 26 H. H. Asquith forms a coalition government in the United Kingdom
- May 31 An LZ-38 Zeppelin makes an air raid on London
- May 31 Indianapolis 500: Italian-born American driver Ralph DePalma accompanied by riding mechanic Louis Fontaine wins from Englishman Dario Resta