(covers information from several alternate timelines)
Tomalak was the commander of the Romulan D'deridex-class warbird that violated the Romulan Neutral Zone in 2366 to rescue the crew of the Romulan scout ship Pi, that had crashed on Galorndon Core. Tomalak denied that the scout ship had illegally entered Federation space by claiming that it had suffered a "navigational failure", and he also denied the possibility of a second crew member since the USS Enterprise-D recovered Patahk from the surface. He was prepared to fire on the Enterprise after Patahk's death, though he desisted after a second crew member, Bochra, was returned safely. (TNG: "The Enemy")
Later that year, Tomalak was part of an elaborate deception involving Alidar Jarok and a supposed Romulan military build-up on Nelvana III. He allowed Jarok's defection to the Federation, and when the Enterprise-D illegally entered the Neutral Zone to investigate Jarok's reports, Tomalak had a pair of warbirds waiting for them. Tomalak boasted how he intended to "dissect the Enterprise-D for every precious bit of information," and then to display its broken hull in the capital on Romulus to both inspire the Romulan armies and serve as a reminder to any other traitors of the state. The crewmembers were to become prisoners-of-war. However, Tomalak was forced to withdraw when the Enterprise-D revealed it had brought along Klingon reinforcements. (TNG: "The Defector")
In a timeline caused by Q and negated by Picard in 2370, Tomalak commanded the Terix, one of thirty warbirds deployed along the Romulan Neutral Zone in response to the appearance of an anti-time eruption in the Devron system. Tomalak confronted Jean-Luc Picard concerning their mutual fleet movements, and each agreed to send a single vessel into the Neutral Zone to investigate. (TNG: "All Good Things...")
Tomalak was played by the late actor Andreas Katsulas.
Tomalak made his first chronological appearance in Vulcan's Heart as a centurion in 2329. At this time, he commandeered a ground car to transport Spock, Ruanek, and Charvanek to a scout ship to escape from the Romulan Neutral Zone.
In the Star Trek Unlimited issue "None but the Brave and Inheritance", set some time after 2367, Tomalak made an appearance in the story "Inheritance" where he was behind the capture of Geordi La Forge and Dr. Leah Brahms in an effort to repair an early Federation technology.
In the IDW Publishing comic series Star Trek: The Next Generation - Intelligence Gathering set in late 2368, he attempted to kidnap Data in order to get assistance with closing an inter-dimensional rift. He finally succeeded on Rete Mire and they were able to close the rift due to the sacrifice of a Romulan Centurion. When Picard asked him why he had gone to such lengths rather than simply asking for assistance and Riker answered by saying "Romulans fear disgrace more than death." Tomalak pointed out that at least one of the Enterprise-D crew understood Romulans.
In the novel The Badlands, Book Two set in 2371, Tomalak had been promoted to High Commander and assigned Centurion Seylok to proceed to the Badlands to capture an artificial quantum singularity. Tomalak had promoted the young man to make it seem like a low priority mission in order to misdirect the changelings.
In the Marvel Comics Deep Space Nine comic series, Tomalak appeared in the issues "Public Enemies, Private Lives" and "Public Enemies, Private Lives, Conclusion" set in 2373 where he was working on a mission with his son together with a group of Maquis. His son murdered the entire Maquis cell and kidnapped Jake Sisko. Tomalak informed Narak that his actions had dishonored their family name. He also appeared in "Day of Honor", when he arrived with Tebok in warbirds to help defend Deep Space 9 against a Dominion incursion later in 2373.
Tomalak made an appearance in the novel The Genesis Wave, Book One set in 2376, he was part of a task force dispatched by the Romulan Empire to aid the Federation. He met with Picard near a Bolian colony.
In the novel Taking Wing, set in 2380, from the Star Trek: Titan series shows Tomalak becoming proconsul of the Romulan Senate after the fall of Shinzon. In Death in Winter, set around the same time, he is portrayed helping protect the planet of Romulus. He is finally arrested and left in the Federation to stand trial after his attempts to help the newly-formed Typhon Pact acquire their own quantum slipstream drive results in the destruction of Deep Space 9 and the Pact and the Federation being pushed to war in the novel Raise the Dawn.
In the backstory to Star Trek Online Tomalak became Fleet Commander after Shinzon's coup, and led a punitive blockade of Remus through 2381. He remained loyal to the Empire when Donatra led several ships and planets in rebellion, forming the Imperial Romulan State (β) in opposition to the Imperial government led by Praetor Tal'aura. Donatra soundly defeated him when he attacked her ships at Xanitla in 2383, and he was further disgraced by the defection of his handpicked second-in-command, Admiral Taris. Tal'aura removed him from command and forced him into retirement at his estate on Romulus. His fate following the supernova of 2387 is unknown.