Lionel Luthor was played by actor John Glover for the entire run of seasons 1-7, becoming a series regular in season 2 through 7. The character originally served as an antithesis to Jonathan and Martha Kent, and soon became the show's central villain. In his final seasons (6-7) Lionel was more or less an anti-hero and father figure for Clark before his death at the hands of his own son in the episode "Descent". Glover returned to Smallville for its final Season (10) as the Lionel Luthor from an alternate world; he serves as one of the many villains Clark must fend off before his first flight as Superman.
Lionel Luthor is the father of destined super-villain Lex Luthor. As a young man Lionel concocted a plan to collect an insurance fortune by burning down the building his parents were living in... he joined forces with thug Morgan Edge, and the two built their respective empires off of their inheritance... Lionel created Luthorcorp... an extremely powerful corporation that has mass influence around the world, and often deals in morally gray areas. Lionel is more or less a self educated man, who learned strategy and his cunning from Machiavelli, Sun Tzu, and often compares himself to a modern day Phillip of Macedonia. He is well versed in Biblical study and Shakespeare, and often quotes his heroes during his lectures or many nefarious threats.
His relationship with his only living son, Lex, is a complex one. From his early days as a father Lionel attempted to 'toughen' up his rather wimpy son in order to prepare him for his destiny. After Lionel's wife Lillian, suffering from post-partum depression, smothers his youngest son Julian in his sleep, Lex steps in to take the blame, shielding his mother from his father's wrath. From that day forward, Lionel and Lex's relationship would be permanently scarred. Lionel was also responsible for the legalization of Clark Kent's adoption papers to his parents Jonathan and Martha Kent, albeit at the price of Jonathan talking the Ross's into selling Lionel their factory, thus driving a permanent rivalry between Jonathan and Lionel for years to come.
Lionel sent his son Lex to Smallville to overlook a minor factory in the Luthorcorp pantheon as a test. Despite Lex's surprising, and often times controversial success, Lionel grows wary of his son's ruthless ambition. Soon, Lionel begins to play a more vital role in Lex's life. However it isn't long before he sets his sights on the mysterious Clark Kent. Once his inquiries into his son's best friend grows to obsession, Lionel begins using the vast resources and his clever ability to manipulate in order to discover the shocking truth about Clark Kent... even going as far as using his friends against him.
When Lex uncovers his father's involvement in his grandparents' deaths he plans to expose him, but a swift and vengeful Lionel frames Lex and plays him off as insane, only to use electro shock therapy to eradicate his son's memory and investigation. As Lex attempts to remember what he lost, he recalls the night of his brother's grisly murder, and reveals the true killer to a shocked and heartbroken father... Lionel tells Lex that had he known, things would have been different... to which Lex coldly replies "Yeah dad, you might've loved me." Lionel soon discovers that he is dying of liver cancer and is soon on trial for his life as Lex, Clark, and Chloe Sullivan join forces to expose his murderous plan against his parents from years past.
Lionel hatches a plan to escape prison by using a powerful, mystical stone to switch bodies with his son, now head of Luthorcorp, but the plan goes awry and Lionel finds himself in the body of Clark Kent. He uses Clark's abilities and plots to soar to new heights of power with his new found gifts... but Clark grows wise to his plans and disocvers a way to switch back. Lionel forgets Clark's secret and any memory of his time within his body, and is also inexplicably cured of his liver cancer. Lionel believes God gave him a seconds chance... and vows to be a better person. His attempts at reform go south when an evil incarnation of Lex provokes the old shark to sharpen his teeth again, and soon, Lionel is back on the path of wickedness to gain control of the other mystical stones that can lead to great power and knowledge. Little does Lex know, Lionel's posseses of one of the stones, and soon after its activation, Lionel's eyes turn white and he begins carving and etching Kryptonian symbols, warning the arrival of an evil presence known as Zod.
Once Lionel returns to full health after his near mental break, he begins, once again, acting in shady areas... mostly in actions against his son's political aspirations to become State Senator. Lionel backs Jonathan Kent in the race against Lex which leads to his son's eventual defeat. But Jonathan was less than thankful as he believed Lionel was attempting to use him as a puppet... it is only after Lionel reveals he is in possession of proof of Clark's power that Jonathan beats his old enemy to a pulp. Jonathan unfortunately succumbs to heart failure after the battle, and Lionel flees the scene. As Martha begins to grieve over her lost husband, Lionel offers both comfort and a chance for her to take her late husband's Senatorial seat. The two become partners and begin a complex relationship that disturbs Clark, as he doesn't trust Lionel, especially not around his own mother. Just as Clark begins to believe Lionel is plotting to use him as a weapon, he is confronted by Brainiac, posing as Jonathan, who orders him to kill Lionel. Once Clark confronts Lionel, Lionel reveals he is the emissary of Clark's biological father, Jor-El, and is there to warn Clark of Lex and Brainiac's plan to unleash the evil General Zod on Earth.
After Lionel assists Clark in his endeavor against Zod, the two begin a trusting, albeit uneasy, relationship. But Lionel doesn't stop dabbling in gray areas as he blackmails Lana into marrying Lex in order to gain information on his son's impending plans. Lionel reveals his true intentions to Clark, and soon finds himself kidnapped by an insane woman in the woods outside Smallville. Lionel escapes with the help of his son, and knows full well that it was a vengeful Lana Lang that was responsible for his kidnapping. Lionel serves as a friend and mentor to Clark for the remainder of his life, but soon is caught in his own deceits once again. After he is exposed as a member of a secret society known as Veritas (joined by Dr. Virgil Swann, Oliver Queen's father Robert, and Jason Teague's father Edward), an organization meant to protect "The Traveler" aka Kal-El of Krypton, Lionel attempts to protect Clark by keeping him locked in a Kryptonite cage... but an over anxious, and prejudiced hired gun of Lionel tortures and nearly kills Clark. After Clark is rescued by his cousin, Kara, he refuses to hear Lionel's justifications, and avoids any contact with him.
Lex becomes obsessed with uncovering the identity of the Traveler, and soon confronts his father in his office. Lex tells him that he was raised in Lionel's shadow, and now Lionel will die in his. After taking hold of Lionel's Veritas necklace, which holds the key to controlling the Traveler, he pushes his father out of the Luthorcorp window, and Lionel falls to his death. With his last breaths, Lionel lied to protect Clark and his secret from a now fully evil and villainous Lex Luthor. Clark is left a final message from his fallen ally, and realizes that, in his death, Lionel joined Jor-El and Jonathan as fathers who died protecting him. Clark went to Lionel's funeral, despite Lex demanding he be the only one there, and after his sworn enemy left the grave site, Clark took a handful of earth and sprinkled it atop Lionel's grave as he had done for his own father years ago.
After his death Lionel was revealed to be the murderer of Oliver Queen's parents as well as the father of Tess Mercer (born Lutessa Luthor), the acting CEO of LuthorCorp. After Tess learns of her true heritage she gains a special Kryptonian "mirror box" which accidentally transports Clark into an alternate reality (known as Earth 2). In this world, Clark was discovered by Lionel in the cornfields, as he originally planned, and he is now a known super villain called Ultraman. Lionel never gained any redemption or remorse in this world and remains his old evil self, with even more ruthless ambition. After Clark manages to use the box to return to his home world, the Lionel of Earth 2 follows him through the wormhole.
Upon arriving in Metropolis of Earth 1, Lionel slowly regains his assets and takes control of LuthorCorp from Tess and Oliver. He also wishes to mend his family line by taking Tess and the clone of Lex Luthor under his villainous wing; but the anger and bitterness in clone Lex's heart makes him turn against Lionel and burn down the Luthor mansion right before Lionel's very eyes. Though Alexander turns against him, soon the clone begins to lose his memories, and it is revealed he is the result of the joining together of both Clark Kent and Lex Luthor's DNA. Lionel refuses to give up on his prodigy, so he gives Alexander, now dubbed Conner Kent, the curse of Red Kryptonite, hoping he would embrace his dark side and "become a god". Clark however thwarts his plan. Tess then uses DNA evidence to prove this Lionel is not the true Lionel Luthor of this Earth, and thus Lionel is hoisted from power.
After being defeated by Clark and Tess, Lionel stands before the real Lex's grave, claiming he would give anything to be with his son again; in that moment, the dark lord Darkseid appears before Lionel. In the show's Series Finale, Lionel has Tess kidnapped and taken to Lex's personal laboratory underneath the charred remains of the Luthor Mansion. There he tells Tess she will serve her purpose by giving Lex her heart, thus completing the resurrection process. She refuses and manages to access a gun from a security guard and shoots Lionel in the chest. With Lionel's last breath, he allows Darkseid to take his soul and body as a vessel should he save Lex. Darkseid pulls Lionel's heart from his body and possesses him. Lex is revived.