Who's This? The masked Mandrake at the top of page 28 in Who's Who vol.XII.
How you could have heard of him: The King worked with the JSA in Star-Spangled Comics special that was part of their return in 1999, and not surprisingly, in the nostalgia-heavy Starman (#66). His son, King Chimera, was a similar character who appeared in the last Justice Society of America series (in #29).
Example story: Flash Comics #25 (1942) by Gardner Fox and Harry Lampert
Who else? With two Killer Sharks to choose from, I decided to wait for a rainy day (or next year's shark week), so we're moving on from the Golden Age. Next stop, Ice Age!