- "One of the most important things that Lex taught me is you can't afford to live in the past." Tess Mercer to Oliver Queen, Committed
Four weeks after Lex Luthor disappeared, Tess went to the Arctic to take command of LuthorCorp and search for signs of their missing CEO. She turned out to be the successor designated by Lex to guard all his interests in case something happen to him and immediately took control of the search project, but was forced to leave the area when they were ambushed by Oliver's team. She then made her way to Smallville to take up residence at Lex's mansion and in the process had her first encounter with Lois Lane. Tess began to look insistently at clues about the Lex's whereabouts and finally ended with a box containing the Crystal of Knowledge and a photo of a footprint that does not belong to Lex.
Tess took possession of all Lex's business, including the Daily Planet and on her first day, she was saved from the burning remains of a bus explosion by her new employee, Clark Kent. After she introduced herself to him, Tess showed herself to be flattered for meeting him, revealing that Lex told her a lot about Clark.
It became apparent that it was no coincidence that Tess was in the bus at the time of accident because she was chasing a girl with metahuman powers to recruit her for her new team of heroes that she was putting together and finally intercepted the prison bus in which the girl was put after Clark caught her.
After Oliver falls poisoned mysteriously, it was revealed that Tess and Oliver knew each other from before and that the two had maintained a relationship in the past. Tess helped to cure Oliver and later he questioned her about how much she had changed and about her alliance with Lex. Tess showed continuing resentment towards him for having cheated her, but however revealed to him that Lionel killed Oliver's parents.
During her research about the crystal she found in the Arctic, Tess tracked down one of Lex's scientist for help and was reluctant to believe that the crystal was an alien technology, but when an experiment on it, sent a beacon attracting an alien princess to Earth, she began to believe it. After the crystal was stolen, Tess became suspicious of everyone who knew about it and later received an email saying that she wasn't ready yet.
At the same time, Oliver tried to rekindle his relationship with Tess and invited her to dinner but however, despite sleeping together, Tess told Oliver that she was not interested in a romantic relationship and rejected him.
Tess continued her recruitment of special people and her search to find out who took the crystal from her. She hired Sebastian Kane to help her find it by questioning Lois since she was her prime suspect and also sent her men after Jimmy, as he was her second suspect.
When the crystal was eventually used, Tess tracked its energy pulse reading to the Kent Farm, where she questioned Chloe about it and started to suspect more of her and Clark. After meeting Faora while she was possessing Lois and learning a bit about Krypton and Kal-El, she called Clark into her office and revealed to him that she knows he was the last person to see Lex alive.
Finally, it was revealed that Tess had made contact with Lex at some point after her arrival in Smallville and that she had been keeping in touch with him through emails.
After Lana Lang returned to town, Tess was approached by her and confronted with the knowledge that Lex was alive and that Tess knew where he was. Tess denied that statement and after she learned that Lana had stolen information about a secret survival project of Lex denominated Project Prometheus, she confronted Lana and started a fight. Lana won the battle and exposed to Tess the nano-transmitter that Lex implanted in her optic nerves. Feeling betrayed, Tess decided to repay Lex by closing his bank accounts and offered Oliver the chance to merge with LuthorCorp.
When Lana went missing, Tess revealed to Clark everything that happened to Lana after awakening from her alien coma. After being confronted by Lex's right-hand man, Regan Matthews, who was revealed to know about the transmitters in her eyes, they got into a fight and Tess managed to killed him. Keen to prevent Lex from getting his hands on the Prometheus suit, Tess tracked it down to Dr. Groll's laboratory and was prepared to kill Lana in order to destroy the suit. However, Lana managed to defeat her and then tried to convince Tess to let go of her desire for vengeance against Lex.
Lex tried to get his revenge on Tess and the other LuthorCorp board members for switching their allegiance to Oliver Queen by hiring Winslow Schott to set a bomb to kill them. However, Tess was out of town to see a surgeon to remove the nano-technology that Lex had inflicted her with.
After returning to town, Tess started to become more suspicious about Clark´s secret and devised a plan to force Clark to expose his powers aboard a jet, and even confided to Clark her deepest secret hoping that in return Clark would reveal his secret to her, but her plan was not successful. After they returned to Smallville, she thanked him for saving her and tried to convince him that he could trust her, stating she isn't Lex.
Tess finally managed to get hold of Lionel's journal and by reading it, surmised that two Kryptonian boys fell to Earth on the day of the meteor shower and soon came to the conclusion that Clark was the one who Lionel called "The Traveler".
Thanks to Jimmy's note, Tess discovered that Davis Bloome was killing criminals and realized that he was the other Kryptonian and attempted to kill him although he survived and then she took him back to the mansion to explore his past with the Luthors. Tess confessed that she believed that Clark and Davis were destined to kill each other and keen for Clark to embrace his destiny but he ignored her.
It was revealed that Tess was in possession of the Kryptonian Orb that Lex deposited in the console of the Fortress the last time and following Clark's refusal to accept his destiny, Tess began to publish articles revealing Davis as the serial killer and began a manhunt against him, knowing that many will die in the process.
Still keen to defeat the threat of Doomsday, Tess found the perfect opportunity to use her team of heroes and sent them after Chloe and Davis, who were on the run. Tess sent her shape-shifting assistant, Eva Greer, to infiltrate Clark's life disguised as Chloe to learned more about him and using the recovered Kryptonian information relayed from Eva, find and destroy the Black crystal to force Clark to confront his destiny and kill Doomsday.
It was revealed that Tess had been taking orders from the Orb who tells her that she will be the savior of Kandor after Clark kills the beast and when Tess returned to the Luthor Mansion after work, she found the Orb missing and that the vault was busted open from the inside and then instructed all of her staff to find it. When Tess discovered that Lois had been in her office, she immediately suspected her and confronted Lois at the Daily Planet basement. Lois accuses Tess of trying to bring an alien nation to conquer Earth but Tess claims she only wanted to save the world. A fight is quickly developed and Lois managed to defeat Tess by knocking her out.
Tess woke up and returned to the Luthor Mansion and after Doomsday was defeated, Tess saw a glowing light outside the mansion. As she approached it she saw the Orb send a burst of light into the ground, out of which came a man standing on the Mark of Zod.