He appeared in flashbacks as the secondary antagonist of the Season 3 episode "Relic".
He was portrayed by the late John Mann.
Lachlan Luthor was born in Scotland in 1916. As a young adult, he met a woman named Eliza and married her. She gave him a son that they named Lionel and moved to America, settling in a poor part of Metropolis called Suicide Slums. Lachlan was extremely abusive towards the boy.
Lachlan had been a low-level criminal who had tried to mug a young woman named Louise McCallum at gun point while passing through Smallville in 1961. Louise tried to fight Lachlan off, but he knocks her down.
Joe the drifter super speeds to her rescue and threw Lachlan against a light post. Deputy Tate shows up and took Lachlan into custody.
A few days later, Lachlan was released from jail, by Deputy Tate. In return for his release, Lachlan agreed to kill the drifter. Lachlan found Joe and Louise as they were embracing one another at the McCallum Farm.
When Lachlan shot the drifter, the bullets just seemed to bounce off him, with one fatally hitting and killing Louise McCallum. Lachlan fled the scene in shock and returned to his life in Metropolis.
Years later, Lionel conspired with his friend Morgan Edge plotted to kill Lachlan and Eliza to collect their insurance money. Lionel agreed to an arrangement with a slumlord of the Suicide Slums district to plan the murder out.
Lachlan and Eliza perished in a building fire that was set by Edge.
Lionel later confessed that he made up the story about his father being a hard-working entrepreneur, descended of Scottish nobility in order to win friends and influence people during his rise among over-achieving Ivy Leaguers, but he did not lie about his death.
Years later, Lex questioned his father about his grandparents after talking to Clark Kent about "a Lachlan Luthor" who was arrested for attempted theft in 1961 to which Lionel said that his parents died in a fire.