- Apr 1 "Big Bang" theory proposed in scientific journal "Physical Review" by American cosmologists Ralph Alpher, Hans Bethe, and George Gamow
- Apr 1 Faroe Islands receive autonomy from Denmark
- Apr 1 Fast bowler Hines Johnson has match figures of 10-96 on debut in West Indies 4th cricket Test win v England in Kingston, Jamaica; first fast bowler to take 10 wickets in a single Test for West Indies
- Apr 3 1st US figure skating championships held
- Apr 3 US female Figure Skating championship won by Gretchen Merrill
- Apr 3 US male Figure Skating championship won by Richard Button
Marshall Plan Signed
Apr 3 US President Harry Truman signs the Marshall Plan to rebuild war-torn Western Europe after World War II, granting an initial $5 billion in aid to 16 European countries
Mack vs Griffith
Apr 4 84-year-old Connie Mack challenges 78-year-old Clark Griffith to a race from home to 1st base; it ends in a tie
- Apr 5 WGN TV channel 9 in Chicago, IL (IND) begins broadcasting
- Apr 7 A Buddhist monastery burns in Shanghai, China, leaving twenty monks dead
- Apr 7 World Health Organization is formed by the United Nations
- Apr 8 Soen Nakagawa & Nyogen Senzaki (Zen teachers) meet in San Francisco
- Apr 9 Massacre at Deir Yassin.
- Apr 10 Jewish Hagana repels an Arab attack on Mishmar HaEmek
12th US Masters
Apr 11 12th US Masters Tournament, Augusta National GC: Claude Harmon shoots a record-tying 279 (−9) to finish 5 strokes ahead of runner-up Cary Middlecoff
- Apr 13 Hadassah Convoy Massacre: Vehicles bringing Jewish staff and medical supplies to Hadassah Hospital and University on Mount Scopus, Jerusalem attacked by Arab forces; 79 killed by bombs, grenades, and sniper fire
- Apr 14 A flash of light is observed in crater Plato on Moon
- Apr 14 NYC subway fares jump from 5 cents to 10 cents
- Apr 14 Stanley Cup Final, Olympia Stadium, Detroit, MI: Toronto Maple Leafs go back-to-back with a 7-2 win over Detroit Red Wings for a 4-0 series sweep
- Apr 14 US performs atmospheric nuclear test at Enewetak Atoll
- Apr 15 1st Jewish-Arab military battle, arabs defeated
- Apr 15 Indian territory of Himachal Pradesh created
- Apr 15 KCPX (now KTVX) TV channel 4 in Salt Lake City, UT (ABC) 1st broadcast
- Apr 16 Organization for European Economic Cooperation (EEC) forms in Paris
Elpidio Quirino President
Apr 17 Elpidio Quirino assumes the Presidency of the Philippines, taking his oath of office two days after the death of President Manuel Roxas
- Apr 19 52nd Boston Marathon won by Gerard Cote of Canada in 2:31:02; his 4th victory in the event
- Apr 19 American Broadcasting Company (ABC) TV network debuts
Chiang Kai-shek President
Apr 19 Chiang Kai-shek elected President of Nationalist China
- Apr 20 NYC hikes subway fare from 5 cents to 10 cents
- Apr 20 Walter P. Reuther UAW President shot & wounded at his home in Detroit
- Apr 21 Basketball Association of America Finals: Baltimore Bullets beat Philadelphia Warriors, 88-73 to take series, 4 games to 2
- Apr 22 WTVR TV channel 6 in Richmond, VA (CBS) begins broadcasting
- Apr 23 KSTP TV channel 5 in St Paul-Minneapolis, MN (ABC) 1st broadcast
- Apr 27 Arab legion attacks Gesher bridge on Jordan River
- Apr 30 Arthur Schwartz and Howard Dietz's musical revue "Inside USA" opens at the New Century Theater, later transferring to the Majestic, NYC; runs for 339 performances
- Apr 30 Organization of American States charter signed at Bogotá, Colombia
- Apr 30 US performs atmospheric nuclear test at Enewetak Atoll
Admirals
Aviators
Gangsters
Generals
Industrialists
Suffragists