"Monster Mash" was written and performed by Bobby Pickett and his buddy Leonard Capizzi. Bobby was a singer/actor who just happened to do a killer (ahem) impersonation of Boris Karloff. Originally he wanted to be an actor, but when he couldn't find any jobs he and his buddy decided to take advantage of his impersonation talent and write and record what became an immortal novelty song. They scraped the money up to record it and pressed a thousand copies. They then shopped it around to various record labels in Los Angeles but were turned down by all of them. Various DJs who were given copies played them on the radio, though, and the rest was history.
It was number one for two weeks over Halloween, 1962. History-buffs might be interested to know that this was the same time as the Cuban Missile Crisis.
"Monster Mash" re-entered the chart in 1970 and then again in 1973, which is when I first heard it.
Boris Karloff evidently thought it was a fun song, as he sang it when he was a guest-host on the ABC-TV variety show "SHINDIG" in 1965.
And here he is doing a live appearance in 2006, which the YOUTube commentator says was his final performance. He died on April 25, 2007.