An injury during childhood left him lame and infertile, and he became an outcast among his village. When Vala Mal Doran was sent to the Alteran Home Galaxy, Tomin found her, believing she was a gift from the Gods. When Vala discovered that she was pregnant, she married Tomin to prevent people from suspecting the child belonged to someone else. A Prior later healed his limp, and he was conscripted into the Ori Army. Vala feared that he would change from the gentle being he already was. Tomin was told by the Prior that Vala's child was not his, but the will of the Ori. He also told Tomin that Seevis was planning to disrupt the launch of the Ori warships, so Tomin was sent to kill him. Vala was there, as well, but she convinced him that Seevis had forced her into helping him. (SG1: "Crusade")
Tomin was part of the first wave of soldiers sent by the Ori. He was present for the birth of Vala's child, the Orici, and explained the meaning of the term to Vala. When Dr. Daniel Jackson, who had ringed aboard the vessel during the earlier battle, attempted to free Vala and kidnap Adria, Tomin thwarted him. Tomin attempted to kill Daniel, but Vala jumped in front of the blast. Tomin was horrified at having shot his wife, but Adria was able to heal her. Daniel took the opportunity to stun them both, then he and Vala were beamed out by the Odyssey. (SG1: "Flesh and Blood")
- "I will not stand by while the holy doctrine of good will and faith that I have sworn to uphold is twisted into a hammer and used to beat people down!"
As the invasion progressed, Tomin became a cunning and ruthless warrior, and a skilled leader. He eventually rose to the rank of Commander. The force he lead was part of the group sent to the planet P9C-882 where SG-1 was testing Arthur's Mantle. Vala Mal Doran was captured, and Tomin was tasked with educating her in the ways of Origin. Her unwillingness to accept Origin eventually exhausted his patience, and he told the Prior he was better suited for combat. The Prior insisted that converting her was his duty, and informed him that combat was no longer necessary as the village would be destroyed. Tomin objected, as some the villagers agreed to accept Origin; the Prior twisted the meaning of the Story of Markon to suit his own needs, causing a crisis of faith for Tomin. At Vala's urging, Tomin made an impassioned plea to the Prior, who he felt was going against the teachings of the Ori. Outraged at Tomin's insubordination, the Prior destroyed the village anyway, simply to demonstrate the power of the Ori to Tomin. Tomin informed Vala of what happened, then sent her down to the planet but refused to go with her, even though he knew he might be killed for his actions. (SG1: "Line in the Sand")
Tomin was seen again on Dakara while SG-1 was searching for the Ark of Truth. His soldiers cornered SG-1, at which point Tomin demanded their surrender, assuring them that their lives would be spared if they complied. SG-1 cooperated in an attempt to get the Ori forces to open the Ark for them, but all it contained was a piece of parchment that crumbled into dust upon being handled. The Prior ordered SG-1 killed, but Tomin objected, having given his word. They got into another heated argument (which their dialog suggests had been happening frequently), and the Prior eventually insisted that Tomin obey. He hesitated, and Dr. Daniel Jackson and Vala Mal Doran convinced him that the Prior could do nothing to hurt them; his powers were being neutralized. With his powers gone and his soldiers unwilling to obey, the Prior grabbed Tomin's staff weapon to kill SG-1 himself. Lt. Colonel Cameron Mitchell managed to grab another staff and kill the Prior first, convincing Tomin and his troops that the Ori were not really gods.
Tomin surrendered himself to SG-1 and returned to Earth with them. On Earth, he aided Daniel in interpreting the visions which had led him to Dakara, revealing that the location he sought was Ortus Mallum, a place in the Alteran Home Galaxy. A mission was authorized to find the Ark. En route, Tomin talked with Teal'c about their past sins. Teal'c matter-of-factly told Tomin that they would never find personal forgiveness, but that devoting their lives to helping others was a small measure of retribution. Tomin expressed great regret for his actions which he admitted he knew were wrong, but he had ignored that fact.
Having arrived in the Ori galaxy, Tomin put SG-1 in contact with Hertis a member of the Anti-Ori underground, who directed them to Ortus Mallum on the planet Celestis. Although he, Teal'c, Daniel and Vala were able to discover the Ark, Tomin and the others were subsequently captured. They were tortured by the Priors until being rescued by Teal'c (who had been left for dead). He was present when the Doci was exposed to the Ark, and translated what he was saying for everyone. With the mission concluded, Tomin decided to stay in his home galaxy, noting that it was the teachings of Origin that gave him strength while he was being tortured by the Priors. Although Vala said that she could not go with him, she told him not to give up on Origin, admitting that the fundamental message was powerful even if the original intentions behind it had been wrong. He and Vala both expressed regret for all of their past actions and both agree that they will spend the rest of their lives trying to make up for them. It is indicated that Tomin became the new leader of the former Ori followers. (Stargate: The Ark of Truth)
- Despite the anti-Ori underground being a secret, Tomin showed a surprising level of knowledge of them, being able to locate Hertis to help find Ortus Mallum.