McGee grew up in Bethesda, Maryland and on the Naval Base in Alameda, California, as his father was a naval officer. His parents bought him his first car the day he turned sixteen - a 1984 Camaro Z28 5-speed. However, he crashed into a bus while trying to figure out how to use the windshield wipers. He got a student pass the day he got out of traction. He received a degree in Biomedical Engineering from Johns Hopkins University and a masters degree in Computer Forensics from MIT. He worked for a time at the NCIS field office in Norfolk, Virginia before being transferred to the headquarters in the Washington Navy Yard which was made permanent at the end of the season 2 premiere episode, See No Evil.His younger sister, Sarah McGee, attends Waverly College and intends to be a writer like her older brother.

Special Agent Timothy McGee, or "Probie", as he is commonly referred to by Special Agent Tony DiNozzo, is part of a major case squad led by Special Agent Gibbs. He gained his field agent status when he was transferred to Washington from Norfolk. In addition to responding to crime scenes and conducting investigations, McGee also serves as Gibbs's computer consultant and sometimes assists Abby Sciuto in the forensics lab when necessary.

As the junior field agent of the team, McGee is often subject to teasing from DiNozzo and Ziva David, often together (though Ziva's teasing is not as mean-spirited as DiNozzo's), and, to some extent, Gibbs. In one episode, Gibbs walked into the lab, found him lying on the floor on his back while working on the underside of Abby's hotbox, and quipped, "Special Agent Goodwrench?" McGee's most notable nicknames come from DiNozzo, either "Probie," due to his junior status on the team, or "McGeek," alluding to McGee's intelligence. DiNozzo uses multiple variations on both of these names to make fun of McGee, some of which are clever but not demeaning (McGPS, McProof, The Great McOz), some which are mocking (McLawyer, McGiggle, McGoo, McNerd, McFlowerPower). Following Gibbs' retirement, McGee is promoted to Senior Field Agent after DiNozzo was promoted to Team Leader, after which Tony displays increased respect for McGee. With Ziva's departure in the episode "Aliyah", McGee is promoted to be Tony's partner in the field, and their relationship in the episode "Truth or Consequences" is shown to be friendly. For example, Tony used McGee's first name, Tim, more often and hasn't called him 'Probie' since season seven began. However, McGee's online gaming persona, "Elf Lord", persists as a nickname which even Gibbs still uses in season seven. However, despite his junior status on the team, it is apparent that, despite teasing, he is on friendly, even familial, terms with every member of the team.

McGee has a younger sister named Sarah who is seen in the episodes "Red Cell" and "Twisted Sister". The two seem to differ greatly in personality, but are very close to each other, as seen in the episode "Twisted Sister."

McGee had an extended relationship with forensics technician Abby Sciuto in season one, but this ended later on. However, despite this, they are still very close and often tease each other about their previous relationship (sexual jokes, suggestive comments, etc.), and in some instances Abby has shown jealousy of attention women have given McGee. In contrast to Tony DiNozzo, McGee has had a few relationships, or possible ones, all of which have begun and ended in the same episode.

When in stressful or uncomfortable situations, as seen in the episode "Grace Period", while trying to deal with the death of a friend, McGee will often retreat to the forensics lab for reassurance from Abby.

McGee owns a dog called Jethro, that was accused of murdering a Petty Officer during the Season 5 episode, Dog Tags. Abby looked after him and named him after Gibbs, but because her landlord wouldn't let her have a dog in her apartment, she forced McGee to take him.

Aside from Gibbs' basement, McGee's home has been shown in the most number of episodes of all the series' characters. Various characters have been inside at one point or another; namely DiNozzo, Ziva David, Caitlin Todd, Sarah McGee, Abby and Abby's previous boyfriend/stalker, Michael.

McGee has a passion for computers and enjoys playing games relating to tactical warfare and covert infiltration. This is shown to be an interest shared by his sister Sarah, as seen in the episode "Red Cell", when she takes over his live war game as he leaves his apartment. In the episode "Broken Bird," his first computer was revealed to have been a Mac SE, followed by a PowerBook, and finally a PC. He uses a Logitech Y-SAE71-SK-2930 keyboard to carry out his everyday work.

In the episode "Dead Man Talking", it is revealed that McGee writes mystery/crime stories, and in the season four episode "Twisted Sister", it is revealed he has published a book entitled "Deep Six". It became known in the episode "Cover Story" that McGee has been successful as a writer, and garnered a fanbase. It was kept a secret from the NCIS team for a long time. Under his pseudonym "Thom E. Gemcity" (an anagram of "Timothy McGee"), he has penned at least two novels as part of the Deep Six series, one published and one in progress. The title of the first book, Deep Six: The Continuing Adventures of L.J. Tibbs (whose name and character are based on Leroy Jethro Gibbs) suggests there have been previous novels, but these have yet to be named, though the cover of McGee's first revealed book states that it is volume five, suggesting four previous novels. The next of McGee's books to be released, and sequel to its predecessor is entitled Deep Six: Rock Hollow. In the episode "Cover Story", Rock Hollow becomes a focus point in that the NCIS team are investigating two murders (both committed because their killer thought they had robbed a convenience store and killed the clerk, as it was written in the book), when McGee reveals everything that has taken place has been part of the book he is currently writing. It is then discovered that the story was stolen from the typewriter ribbon McGee threw away, and that Abby could be the next victim.

McGee's writing has given him enough fame to be feted as a minor celebrity and he can enjoy perks such as VIP clubbing, as seen in the episode "Friends and Lovers". McGee has ostensibly used the profits from his writing to purchase a Porsche Boxster (seen in the episode "Twisted Sister") and an Armani leather jacket (seen in the episode "Dead Man Walking").

The characters in McGee's books are actually based on the NCIS team, with Leroy Jethro Gibbs as L.J. Tibbs, Anthony DiNozzo as Tommy, Ziva David as Lisa, Abby Sciuto as Amy Sutton, Jimmy Palmer as Pimmy Jalmer, and himself as McGregor. This displeases some of the NCIS team, and McGee is victim to constant torment from Ziva and DiNozzo for many episodes thereafter. Palmer seems especially unhappy with his portrayal, which seems to include some acts of necrophilia, and when Gibbs learned that his character Tibbs' love interest was also a Colonel, Gibbs seemed surprised and unhappy about his private life being a part of McGee's books.

McGee is also shown to be an expert tracker, being able to find a cadaver without the assistance of dogs by reading changes in the land and observing bird droppings. He revealed he was a scout, which occupied his time while his father was deployed. He even said "I love the outdoors", much to the chagrin of the more city-like DiNozzo. However, at least twice McGee has encountered poison oak and suffered severe rashes.

Whenever he is seen writing, McGee is shown to be left-handed, a trait he shares with Gibbs's former mentor and partner, Mike Franks as well as Ducky's former medical assistant, Gerald Jackson.