Before Season 5
Lehman was a specialist who worked with patterns in the Pentagon where he was recruited by "a special foundation", along with other bright minds. This is where he met Vaughn's father. The team was given one page of a book named The Fifth Prophet that had a 15th century text with an unbreakable code. After several years, the team broke the code. Not even a week later, all the people involved with the project began to die in mysterious deaths and Lehman and Vaughn's father knew they were next. Vaughn's father and Lehman changed their names and disappeared.
After Vaughn was captured by Prophet 5, he and Sydney were contacted Lehman to try and uncover the secrets of Prophet 5. Lehman told Sydney about his past and also informed them that he had a lead on the book, but his attempts on acquiring the book were futile. Lehman warned the two what they were getting into, but they took the risk anyway to go search for the book.
When he recovered the book, Vaughn met with Lehman at a train track. Lehman told Vaughn that he'd be able to decode the encryption, but it would take a while. Immediately afterwards, a car with Gordon Dean and one of his guards inside came by. Lehman tried to escape from Dean, but he took out a pistol and shot him in the back, shortly before one of Dean's henchmen riddled his back with bullets. Vaughn wasn't fortunate either; Ivan Curtis appeared and shot him multiple times with a submachine gun.