At the beginning of the series, Will was just Sydney Bristow's friend and a local reporter with a Los Angeles newspaper. Following the death of Sydney's fiancé, Danny Hecht, Will began pursuing leads surrounding Danny's bizarre death. Sydney, knowing the truth (Danny was killed because SD-6 found out he knew Sydney was a spy) asked Will to cease his investigation to protect him from the same fate. However, after a short time, Will went on to investigate and soon found himself wrapped up in Sydney's world of espionage. After a certain time, Will had acquired enough facts to write an article about SD-6 and handed it to his editor to publish if he didn't contact her within the next 7 days. In that time, Tippin had planned to see the unknown voice in Paris in person, who had supported him in the last weeks with hints about SD-6 in general and Jack Bristow in particular. Having arrived there, he instead learns about the true identity of Sydney Bristow. Since he is now in danger because of his knowledge of SD-6, he is brought into a safehouse of the CIA in Los Angeles, only to be kidnapped out of that safehouse to Taipei by Julian Sark. Sydney, Jack and Vaughn had to get there to rescue him. In order to discredit Tippin's story—and at the same time, save him from being killed by SD-6—Jack arranged for him to appear as a drug addict in order to condemn his trustworthiness as a newspaper reporter.
In the first half of the second season, Will was recruited as a private researcher for the CIA. Utilizing his tenacity and journalistic instinct, Tippin researched the secret government experiment Project Christmas for Michael Vaughn. That experiment was designed to use standardized tests given to school children to determine potential future CIA recruits. The Soviets became aware of the project and sent KGB operative Irina Derevko on an undercover mission to determine the specifics so they could enact their own version of the program.
Will briefly dated Francie Calfo and shortly after, unbeknownst to him, Francie's genetic doppelganger Allison Doren. Will discovers the ruse by finding Provacillium in Sydney's medicine cabinet, medication taken by gene therapy patients. At exactly the same time, Allison looks through Will's work and finds out Will is researching an "Allison Georgia Doren." Allison realizes Will is close to knowing the truth, and attempts to kill him, nearly succeeding in doing so. Upon being nearly killed, he was quickly shuttled into the Federal Witness Protection Program, removing him as a regular character for season three. However, he did have a brief reappearance after he was sought out by Sydney and the two had a brief tryst. During this reunion mission, Will finally encounters Allison and kills her for the murder of Francie, after which he returns to private life.
Near the end of the 5th season, in the episode "There's Only One Sydney Bristow" (also the series' 100th episode), Tippin is kidnapped by Anna Espinosa and used as bait for Sydney. During the rescue mission, a sample of her DNA is stolen. Later, it is discovered Will's captors (Prophet Five) have implanted a miniature bomb in his head, set to go off if Sydney doesn't deliver a Milo Rambaldi artifact to Anna aboard a train in Europe. Will accompanies Sydney on the mission, which is once again a trap to gather more genetic material from Sydney (ultimately used to transform Anna into a genetic duplicate of Sydney in the same way Francie was replaced by Allison). Will rescues Sydney and Sydney disarms the bomb with a remote control. During this mission, Will informs Sydney he was planning on proposing to his girlfriend before he was kidnapped and asks Sydney to be his best man, to which she agrees with enthusiasm. This is the last we hear of him.