Lenny was a security guard for a bank in Dodge City, Kansas.
While Lenny was working at the bank he was working in it was robbed by Dave Mather, who was actually a ghoul. Lenny and Dave ended up starting a firefight between the two. Jack Kline, who had tracked Dave to the bank appears and attempts to end the firefight by unleashing a telekinetic blast which hits both Dave and Lenny who had emerged from the bank to help. Lenny is inadvertently blasted headfirst into a metal pole, fracturing his skull and killing Lenny instantly. Though Castiel tries to heal Lenny, he is too late.
After Dean and Sergeant Joe Phillips kill Dave, Dean pins Lenny's death on Dave to protect Jack. Jack is devastated by his accidental killing of Lenny, particularly when he learns that Lenny had a family. Though Lenny's death convinces Jack that he really is a monster, Jack's reaction to it finally convinces Dean of the opposite, that he was wrong about Jack who leaves rather than risk hurting anyone else.
Lenny's death continues to haunt Jack for the rest of his life, with Jack occasionally experiencing flashbacks to it in moments when he is particularly worked up with guilt.