Detroit, Sept. 10, 1960: Mickey Mantle hit what many still believe to be the longest home run ever out of the park at Briggs Stadium (renamed Tiger Stadium the following year). Not ‘officially’ measured until 1985 (by using Pythagorean Theorem!), the hit went 643 ft. before landing in Brooks Lumberyard. But some maintain the longest HR goes to fellow Yankee Babe Ruth in 1926, Also hit in Detroit (Navin Field at that time), this ball traveled 626 ft., landed, and rolled for another 200 ft. before being swooped up by a lucky kid on a bicycle. You decide.
Source : Michigan Historical Society Facebook Page, September 10, 2014.