The history of the notorious murderer Mister Zero prior to 1942 has yet to be fully revealed.  The first known case began when two ordinary looking repairmen arrived at the office of J.S. Thorndyke, a prominent business executive.  Instead of actually repairing the phone, the men installed a small firearm that discharged into Thorndyke's head when he answered a random phone call.  The men - agents of Mister Zero - reported back their success, which Mister Zero described as Method #376.  He offered the men an imported cigarette, which quickly killed them from laced toxins - described as Method #78 - as punishment for errors made during the Thorndyke case.  Mister Zero, it is revealed, runs one of the most methodical assassination operations in the United States.

His next target is revealed is to be baseball player Lefty Burman, who gamblers have contracted Mister Zero to eliminate.  He tasks two men with Method #145 - revealed to be an explosive baseball that would detonate on impact, killing Lefty.  At the game, Johnny Chambers (secretly Johnny Quick) is taking photos during the game and recognizes one of the henchmen giving the umpire a ball.  Suspicious, he deduces that the ball must be a threat and quickly changes into his heroic identity, saving Burman and rounding up the two thugs.  Cornered, one of them shares the address of Mister Zero, expecting to lead Quick into a trap.

Arriving, Johnny flips on the light and finds an empty house but then feels weak and sleepy.  Method #45 is to coat a light bulb with an sedative that turns into a gas when heated.  Mister Zero emerges from seclusion and take the unconscious Quick to his lair as an education for him many henchmen.  Later, he presents Quick frozen in a block of ice for Method #971.  As he lectures his minions, Mister Zero stresses the importance of the maddening array of techniques, making his murders impossible to predict.  Ignored for the moment, Quick begins to fidget and breathe heavily, using tiny opportunities to melt the ice.  Eventually, he is mobile enough to take flight - albeit encased in ice - and engage Zero's men. 

Initially successful, Johnny is eventually overwhelmed by sheer numbers and bound again.  This time Mister Zero uses Method #6 - flammable chemicals - leaving Johnny to burn while he robs the Van Eslen estate of its jewels.  In the meantime, Johnny manages to roll into the water of the melted ice, preventing the flames and giving him time to free himself.  He races ahead to Van Eslen's and alerts them of Zero's plan.  Zero arrives to know resistance but when he finds the safe unlocked, he delightedly reaches in.  A steel trap grabs his fingers and Quick emerges, quickly subduing the elder criminal and leaving him wrapped up for the police (More Fun Comics #75)

Give his advanced years (or presume appearance thereof), it seems unlikely that Mister Zero lived into the 21st century.  His finale fate on Earth-Two is yet to be revealed.