Jack Finnegan is the father of doctor John "Finn" Finnegan, who lives in Malibu, California. Jack paid Finn a surprise visit and learned Finn was about to marry Steffy Forrester, the co-CEO of the Forrester Creations fashion dynasty. Jack supported Finn when Finn told Steffy that Jack had adopted him, but seemed nervous at Finn's wedding to Steffy. Jack confronted a veiled woman who showed up for the ceremony, only to find out it was Pam Douglas, Steffy's great-aunt. However, after the vows, Jack's jaw dropped when Finn came in with Sheila Carter, whom Finn introduced as the biological mother he had just met. Jack earned the ire of his wife, Li Finnegan, as he revealed that he'd known Sheila was Finn's real mom because Sheila had approached Jack when Finn was a baby. Jack was horrified to learn that Sheila was a convicted criminal, whose crimes included causing Steffy's mother, Taylor Hayes, to be presumed dead after Taylor and Sheila struggled over a gun.

Jack joined the Forresters in discouraging Finn from having any contact with Sheila and demanded that Sheila stay away. However, Jack found himself between a rock and a hard place when Sheila threatened to go public with the fact that Jack and Sheila had had an affair behind Li's back, which had produced Finn; Jack had adopted his biological son. Jack felt he had no choice but to honor Sheila's request to hold Finn's son, Hayes Forrester Finnegan, while Steffy was out of town, and cajoled Finn into allowing it. But when Steffy came home unexpectedly, Jack watched as Steffy threw Finn out of the house.

Jack took Sheila's words to heart as she changed her mind and suggested that Jack tell Finn the truth about being his biological father. Jack indeed confessed to Finn and Li, but faced their rejection after they couldn't stomach his decades-long deceit.

A tearful Jack visited Steffy and her family when Finn was killed by Sheila. It was later revealed that Jack and Li were "no longer together." Jack eventually learned off-screen that Finn actually survived his shooting; when, sometime afterwards, Sheila summoned defense attorney Jack to see her in prison to work on developing a case for her release, Jack refused and wanted nothing to do with Sheila.

Jack returned in 2024 for the comeback concert of former rocker Tom Starr, where Jack ran into Li and engaged her in small talk. After Tom mysteriously died at the end of his set, Jack repeatedly texted Li.