Leonard was a small time crook. Upon getting a tip where to score a load of diamonds, he broke into S.T.A.R. labs, specifically that of Doctor Freis. There he found the diamonds powering a machine used to keep the doctor's wife alive. He steals the diamonds and is found by Doctor Freis. The two struggle and cause an explosion. Both breath in a caustic experimental gas. Len sees the Doctor's skin start to turn to ice, and escapes. As Len arrived home, he was startled by an alley cat. The suprise caused him to emit a beam and freeze the cat. Soon Len finds that he can channel this power through objects. He also found he was no longer affected by the cold.
The police had figured out who committed the robbery and surrounded Snart's apartment. As Len prepared to fight back, a man stepped out of his closet and told Snart to follow him. Snart emerged in the offices of Vandal Savage who offered him fellowship in his Legion of Doom.