Tomalak first crossed swords with Captain Picard and the crew of the Enterprise when they discovered him searching for the pilot of a Romulan ship which had "accidentally" crossed into Federation territory. Tomalak insisted there had only been one officer on board the ship, which had crashed on Galorndon Core. The Enterprise had retrieved an injured survivor but refused to return him to Tomalak without searching for Geordi LaForge, who was still missing on the planet. Tomalak invaded Federation space in response and, on learning the Romulan had died of his wounds, threatened retribution. However, he was mollified when Picard revealed they had recovered a second survivor of the one-man ship, who had worked with Geordi to survive. Tomalak smoothly claimed he had been misinformed and was escorted out of Federation space after retrieving the crewmember.
Tomalak was later involved in a plan to discredit the pacifistic Admiral Jarok, which involved feeding him false reports which suggested the Romulans were constructing a base in the Neutral Zone to attack the Federation from. As expected, Jarok defected to the Federation to warn them, but when the Enterprise arrived at the planet in question, they found no sign of a base and Tomalak waiting in ambush with two ships. Tomalak dreamed of taking the Enterprise back to Romulus as a trophy but backed down when three Klingon ships appeared to back up the Enterprise.
After being knocked unconscious, Riker woke up to be told he had lost sixteen years of memory and was now captain of the Enterprise, meeting the Romulans for peace talks. Riker was concerned that Tomalak was the Romulan ambassador and that they were escorting him to the secure Outpost 23. He quickly realised this was an illusion. Tomalak told Riker that the Romulans had captured him and engineered the scenario in order to discover Outpost 23's location. However, this turned out to also be untrue, with Tomalak himself being part of an illusion created by an alien orphan, Barash, in order to trick Riker into staying with him.
All Good Things
In an alternate timeline created by Q causing a rupture in anti-time, Tomalak led a Romulan taskforce to investigate the rupture, located in the Neutral Zone. Picard suggested one ship from each side investigate. When Tomalak learned Picard had no authorisation for the planet, he noted he liked it and agreed.
In the Typhon Pact novels, Tomalak remained loyal to Praetor Tal'Aura after the Imperial Romulan State led by Empress Donatra split off from the Romulan Star Empire. He helped negotiate the Star Empire's entry to the Typhon Pact and frame Donatra for an assassination attempt on Ambassador Spock, resulting in the Romulans being reunited. However, the Tzenkethi arranged for Tal'Aura to die and be succeeded by the significantly less jingoistic Praetor Kammenor.
Unhappy that Kammenor favoured friendly relations with the Federation, Tomalak and Sela conspired with other elements of the Typhon Pact (notably the Breen and Tzenkethi) to steal components from a Dominion ship yard in order to construct a slipstream drive. Although the plot resulted in the destruction of Deep Space 9, their main aim was unsuccessful and Tomalak was taken into custody by the Federation, with Kammenor happy to leave his fate in their hands.