- "My breakfast burrito is about to say hello!"
- —Fornell after discovering that his ex-wife is an IRS agent in the Season 10 episode, "Devil's Trifecta".
In addition, Fornell often works alongside the NCIS team if a case crosses over into the jurisdiction of the FBI.
At some point, Fornell joined the FBI and he and Diane later divorced.
NCIS Season 1
Fornell made his debut appearance in the NCIS Season 1 premiere episode, Yankee White (episode) where he attempted to take the lead in the investigation into the circumstances surrounding Commander Ray Trapp's death. He failed to do so and was later kicked off Air Force One by Special Agent Anthony DiNozzo much to Fornell's own annoyance.
However, Fornell was later brought into the investigation and as such, was given copies of the findings that Forensic Scientist Abigail Sciuto and Medical Examiner Donald Mallard had discovered and after Leroy Jethro Gibbs had taken down the terrorist responsible for killing both Trapp and Major Timothy Kerry,
Fornell was later seen giving a press conference where he discussed the case and also revealed that the terrorist's body would be removed to the Armed Forces Institute of Pathology.
He even personally came to NCIS Headquarters to inform Gibbs and Kate that one of the persons of interest they were searching for into the death of a Navy Commander was actually a terrorist, not a drug dealer as NCIS had originally thought.
He later ordered security to be tightened after discovering that the terrorists were planning an attack and left, only for Gibbs to state that he believed that Fornell had target fixation, meaning that Fornell was so fixated on strengthening the security of potential targets he was failing to see the bigger picture.
His third appearance came in the episode, Enigma (episode) where he attempted to arrest Gibbs's former C.O, having believed that the man was responsible for the death of an agent under Fornell but Gibbs was successful in proving that Ryan was innocent and that the true killer was caught.
His fourth and final appearance of Season 1 came in the Season 1 finale episode, Reveille (episode) where he grudgingly arranged a meeting between Gibbs and the newly identified terrorist, Ari Haswari who was responsible for the NCIS Hostage Crisis.
NCIS Season 2
Fornell's first official appearance of Season 2 came in the episode, The Bone Yard (episode) where he and Gibbs worked together to expose a mole in the FBI who was giving confidential information to a mobster which had resulted in the death of an undercover agent. Gibbs and Fornell even went so far as to fake Fornell's death in hopes of distracting the mole which worked.
It was later revealed that the mole was Agent Charles, Fornell's partner who upon being exposed attempted to kill everyone at the meeting site, only for Charles to die when he was shot in the head by a sniper.
With the matter resolved, Fornell resumed working as an FBI agent again although DiNozzo, Kate and newest Timothy McGee who were members of the Major Case Response Team Gibbs led were left stunned as to how Fornell had even faked his supposed death in the first place.
Fornell returned in Conspiracy Theory (episode), aiding the team with an investigation into the supposed suicide of a Petty Officer whose boss was also being investigated for allegedly accepting kickbacks.
Fornell's final Season 2 appearance came in the Season 2 finale episode, Twilight (episode) where he helped NCIS in the investigation of the reappearance of the team's old enemy, terrorist Ari Haswari.
NCIS Season 3
Fornell appeared in the Season 3 episode, Kill Ari Part 2 (episode) to give Gibbs some possible information into why Ari Haswari had returned to the States.
Fornell's next appearance in Season 3 came in the episode, Under Covers (episode) where he helped NCIS with an undercover operation concerning a possible assassination due to occur at the Marine Corps birthday ball.
Fornell's Season 3 appearance was in Frame-Up (episode) where he arrived at NCIS to take over the investigation and determine why NCIS Special Agent Anthony DiNozzo had been framed for murder. He also informed Gibbs that DNA taken from the leg of a Jane Doe matched that of DiNozzo's although the NCIS team were later successful in apprehending the true mastermind.
NCIS Season 4
Fornell's first appearance of Season 4 came in the episode, Escaped (episode) where Gibbs who had retired from NCIS returned to Fornell to find out the mastermind for a robbery that Petty Officer Derrick Paulson had allegedly committed.
Fornell returned in Smoked (episode) where he was assisted by the NCIS team in an investigation into a possible serial killer, only for Fornell and everyone involved to get a huge shock when they discovered the true killer.
Fornell's next appearance was in the episode, Brothers in Arms (episode).
His final appearance in Season 4 occurred in Angel of Death (episode) where he informed Gibbs that the CIA had scheduled all NCIS agents to take polograph tests.
NCIS Season 5
His final appearance in Season 5 came in Internal Affairs (episode) to interrogate the members of the main NCIS Major Case Response Team to determine who had killed international arms dealer, René Benoit.
NCIS Season 6
Despite not appearing in Toxic (episode), Fornell's name was mentioned.
NCIS Season 7
Fornell's first Season 7 appearance came in Jack Knife (episode) where he and NCIS worked together to take down an illegal cross-country trafficking ring.
His second and final appearance was in Moonlighting (episode).
NCIS Season 8
NCIS Season 9
NCIS Season 10
Fornell appeared in the Season 10 finale episode, Damned If You Do (episode) where he was seen through a sniper rifle that Gibbs was wielding.
NCIS Season 11
Fornell appeared in the Season 11 premiere episode, Whiskey Tango Foxtrot (episode), having been called in to investigate a hotel bombing that had left Thomas Morrow injured and had killed the current Secretary of the Navy, Clayton Jarvis.
When Mendez attempted to set off another bomb, Gibbs responded by shooting Mendez, killing the man instantly with the shot hitting Fornell in the ass.
After the shootout, Fornell was wheeled away and while doped on painkillers, kept complaining about how Gibbs had shot him in the ass with Gibbs remarking that it was merely a surface wound.
Fornell later returned in the Season 11 episodes, Crescent City (episode) and Crescent City Part 2 (episode), the backdoor pilot episodes for NCIS: New Orleans where he helped Gibbs's team solve the murder of a former NIS agent-turned-Congressman, Dan McLane with Fornell even meeting Dwayne Cassius Pride, an NCIS Special Agent who was the head of the NCIS: New Orleans field office team.
NCIS Season 12
NCIS: New Orleans Season 1
Fornell appeared in the NCIS: New Orleans Season 1 episode, "It Happened Last Night".
As one of the most senior agents associated with the FBI, Fornell is highly skilled in interrogating and also using firearms.
Fornell is a highly skilled interrogator capable of getting whatever information he requires out of whatever suspect he is interrogating.
- In the Season 1 premiere episode, "Yankee White", William Baer was heard calling Fornell "Tom" which hinted that it was "Tom Fornell" although the NCIS writers presumably later changed it to "Tobias" to avoid confusion between Fornell and Thomas Morrow who was NCIS Director at the time.
- Used a Glock 17 as his service weapon before presumably switching to a SIG-Sauer P228 which was seen in the Season 11 premiere episode, Whiskey Tango Foxtrot (episode).
- Fornell is the only recurring character to have appeared on every season of the show so far and as far, is the longest-recurring character of NCIS as well.