- "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".
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.
Fornell's second appearance came in Seadog (episode) where he took at former Secret Service-turned-NCIS Special Agent Caitlin Todd by remarking that she'd joined "the cowboys"- NCIS. 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 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.
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. With Fornell's help, the NCIS team eventually captured the Petty Officer's two killers.
Fornell's final Season 2 appearance came in the Season 2 finale episode, Twilight (episode).
Fornell's first Season 6 appearance came in the Season 6 episode, Nine Lives (episode).
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.
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. He also acted as a bodyguard for Tomás Mendez who was later revealed to be working for the team's enemy, terrorist Benham Parsa.
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.
- 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.
- Has a young daughter named Emily Fornell.
- Used a Glock 17 as his service weapon before presumably switching to a SIG-Sauer P228 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.