Little is known of King Leo before his first documented case in the winter of 1947. It is known that he was a circus performer and animal trainer who, along with a ring of his colleagues, were dismissed from the Reilly Circus for fleecing the customers.  His name was known around the New York area, including by Ted Grant, who was familiar with him in this time frame.

In the first documented case, police were confounded chasing a car of criminals firing at them.  Witnessed by Ted Grant, the chase is joined by Wildcat who catches up to the car on his Cat-cycle and is astonished to see the car is driven by a dog.  The dog talks and is evidently shooting at the police behind it.  When the car swerves out of the road, Wildcat catches up by now finds a cat instead.  Dumbfounded, Wildcat is caught off-guard by King Leo and his men.  Wildcat recognizes Leo but in the ensuing struggle is subdued.  Hearing arriving police sirens, the villains depart, leaving Wildcat with the dog.

Later on, a flock pigeons holding guns descend on a jewelry store and announce a stick-up.  Before the shocker jewelers can react, the pigeons seize claws full of jewels and depart.  When one of the men tries to stop them, he is shot, seemingly by the pigeon.  When the birds arrive back at King Leo's hideout, it is revealed that King Leo had fired the shot from a remote location.  Overhearing all this, Wildcat bursts in, having tracked  the criminals back to their lair.  King Leo, an animal trainer by trade, unleashes several large cats and leaves Wildcat to their mercy and announces his plans to target a circus next.

Finding some meat, Wildcat manages to distract the cats long enough to escape.  In the interim, Leo and his gang have arrived at the Reilly Circus,  In their old circus costumes, they go unnoticed by the patrons until they release lions into the crowd.  As the prepare to rob the place blind in revenge, netting drops from the ceiling and snags the lions.  Wildcat follows in short order and caught off-guard, King Leo and his gang are quickly defeated and handed over to the authorities.  The orphaned dog from the car chase goes home with Ted Grant (Sensation Comics #72).

The activities of King Leo after 1947 have not been recorded.