The first television coverage of the St. Patrick's Day Parade grew unexpectedly out of local New York station WPIX's effort to test a new camera and microphone on St. Patrick's Day in 1949. 

Joe McCarthy described the event in American Heritage (February 1969, Volume 20, Issue 2):

When the parade was first shown on television, in 1949, it was allotted one hour of airtime. During the telecast, the station was flooded with telephone calls from people who wanted the parade kept on the air longer. The announcer was told to keep the show going for an additional half hour but the calls continued to pour into the station. In the end he and the cameraman were at their posts on Fifth Avenue for six hours, until the last of the high school bands was making its way past the reviewing stand in the darkening twilight.