He was an artist for EC comics in the early fifties, mainly for their war titles such as "Two-Fisted Tales", and was also one of the five original MAD artists (along with Harvey Kurtzman, Will Elder, Wally Wood and Jack Davis), contributing art to nine of the first ten issues, and later he became the main artist for the MAD imitation magazine, the much maligned "Cracked".
When I was a kid in the sixties, I was never much of a fan of Cracked (nor the other MAD imitation magazine "Sick"), realizing the writing was usually weak and unfunny, and the artwork was below par. I vastly preferred (and collected) the essential MAD...
...But I WAS a fan of John Severin's artwork, so I would pick up an occasional issue of Cracked over the years.
What I DID enjoy the most about Cracked though, and collected, were the sporadic paperbacks that featured beautifully painted covers by Severin, featuring their own Alfred E. Neuman imitation, side-splitting Cracked Mascot, janitor Sylvester P. Smythe:
John Severin did only five of the Cracked paperback covers, all of which I've included here: