Introduced as a drink in 1966 by Mott's or Lord Mott's Canning as known then. It was a combination of clam broth and tomatoes. They even secured a trademark for the Clamato brand.
Today the drink is used as a mix for alcoholic drinks or cocktails mainly in Mexico and Canada but not so much in the United States. They also have Beefamato which you guessed it is a cross been beef (broth) and tomato juice. Again, it's main use is for cocktails.