As District Attorney for Llantano County in the 1990s, Henry "Hank" Gannon was an extremely smart and ambitious workaholic, obsessed with the enforcement of the law. Hank grew up on the streets of Chicago. He was a rebellious, smart-ass teenager, until pushed by his mother -- and by a few good teachers who noticed him in school -- to use his brains to get ahead in the world. By senior year, he was a national high school football player, dubbed by the local papers as "The Cannon" Gannon. Hank attended Midwestern University where he met and married fellow student Nora Hanen. For many years, Hank never knew that, while in school, Nora slept with his younger brother, R.J. After Nora gave birth to a daughter, Rachel, her marriage to Hank became strained as constraints on their time and money intensified. When Rachel was five, Hank and Nora divorced. Hank was extremely bitter when Nora was granted full custody of their daughter.

In 1992, the Gannons all arrived in Llanview, Pennsylvania, and Hank and Nora found themselves at opposite sides of the courtroom: Hank was the DA, and Nora was a defense attorney. Hank tried and won a 1992 conviction when Sarah Buchanan was charged with the murder of Carlo Hesser. (Sarah was found innocent when Carlo's niece Stephanie confessed to the crime. Later, Carlo was revealed to have staged the entire escapade.) Going head to head with his ex-wife, Nora, Hank tried the Marty Saybrooke rape case in which several fraternity brothers, including Kevin Buchanan and Todd Manning, were charged with sexually assaulting Marty. Marty recanted her testimony and exonerated Kevin, while the remaining three fraternity brothers were found guilty in 1993 after Marty, Kevin and Hank's daughter, Rachel, obtained the evidence that drove Todd to plead guilty.

In 1994, Hank Gannon prosecuted Dorian Lord for the 1976 murder of her husband Victor Lord and won the case, although later evidence caused the verdict to be overturned. In 1995, he presented the state's evidence against police officer Andy Harrison in a case involving possession of stolen gun, intent to sell, and the murder of "Ice," a local Llanview teenage gang member. The charges were eventually dropped. That same year, R.J. Gannon, Hank's bad-boy brother, arrived in town and set out on a mission to destroy Hank's career. He chose as his weapon a doctored videotape taken when Hank was a student at Midwestern University that allegedly showed him standing by as a murder was committed. Hank resigned from office, but his friend Bo Buchanan located the real videotape, exonerating Hank, whose pristine reputation was restored.

On Valentine's Day, 1995, Hank married his longtime love, nurse Sheila Price. Their muddled marriage ended in 1996, however, after Sheila told Hank she was pregnant. Hank responded negatively to his wife's condition, causing Sheila angst. Days later, she miscarried the baby, then left Hank and went to London to stay with her brother Dr. Ben Price. In 1996, Hank was determined to find evidence to jail the recently resurrected mobster Carlo Hesser. When Carlo was shot in the head aboard a yacht in late 1996, Hank prosecuted Antonio Vega for the crime, but lost the case in 1997 when it was revealed that Alex Hesser, Carlo's former wife, was the culprit. In 1998, Hank thought he'd be prosecuting Todd Manning again, this time for the murder of Georgie Phillips. Only it turned out that Hank's daughter, Rachel, was the real culprit.

Hank dated Carlotta Vega for a while, but when he asked her to marry him, she turned him down and their relationship ended. Hank then started seeing Det. Lisa West of the Llanview Police Department. Hank was Police Commissioner Bo Buchanan's best man in his wedding to Lindsay that never happened. Hank prosecuted Lindsay's son, Will Rappaport, in his embezzlement case. Bo presented evidence to Hank that Asa Buchanan set him up. Hank couldn't quite give into Asa setting Will up, so he offered Will a plea bargain. But Judge Simmons threw out the plea bargain and gave Will the maximum sentence.

Nora accompanied Will on the train taking him to Statesville Prison to start serving his sentence when the train derailed and Nora was presumed to have died. Hank was one of the many people who thought Nora had died in the train crash, not suspecting that she was being held by Lindsay's brother-in-law, Dr. Colin McIver. Hank tried to help Sam Rappaport determine whether he was the father of Nora's son Matthew. Hank was with Sam when he went to Colin's house looking for Nora and was convinced by Sam that Colin is behind Nora's disappearance. Hank was thrilled when Nora finally returned to Llanview, and he was the only one she remembered at first because she had lost most of her memory.

In 2001, Hank prosecuted Sam for Colin's murder, but when Nora remembered on the stand that she was the one who had killed Colin, he recommended that no charges be filed against her. Later that year, a young woman named Keri Reynolds came to town claiming that she was Hank's daughter. It turned out that Hank's brother, R.J., was really Keri's father. Keri was born when R.J. was in jail and her mother never wanted R.J. to know that she existed. Over the years, Hank gave Keri's mother money for her support, but never let his brother know that he had a daughter. R.J. was furious with Hank when he finally learned the truth about Keri.

The Gannons were devastated when Keri committed suicide in 2003, leaving behind her young daughter Jaime. After a long run as District Attorney, Hank resigned his post in 2003 due to bad press surrounding the Llanview Police Department's handling of the Mitch Laurence case. He began teaching law at Llanview University but later moved to Chicago to be near his daughter Rachel. In Chicago, he was teaching law and had taken up coaching college football.

In April 2009, Hank paid a brief visit to Llanview to support Nora, who was struggling to cope with her son Matthew's paralysis following a car accident. Hank enjoyed visiting with his best friend Bo Buchanan, and questioned him about Nora and Clint's relationship. Sensing that Nora needed more support than he could provide, Hank brought their daughter Rachel to stay with Nora as he returned to Chicago.