It isn’t surprising you don’t remember his from Batman. He wasn’t credited on Batman on his first appearance. Just a ? on the credits. This was on Me TV just a few weeks ago. I read that he hated that role because no one could see his real face and he refused to be credited. I think the second time he was on his name was shown in the credits.
His name appeared in the closing credits of the second half of the two part "False Face" episode. There was a lot of speculation at the time as to which Hollywood celebrity was playing False Face (EVERYBODY in Hollywood wanted to appear on "Batman."). One rumor contended that it was Frank Sinatra behind the mask.