A seemingly routine job taken by her husband, former NYPD cop Michael McMahon, 57, unraveled into an almost 10-year nightmare, resulting in him becoming the first private investigator convicted of spying for the People’s Republic of China (PRC) in the US.

In one scene which could have been plucked from Hollywood thriller, a dozen FBI agents surrounded their New Jersey home in the fall of 2020.

“From the moment the FBI knocks on your door, your fight-or-flight instinct kicks in,” said Byrne, who played Lily Walsh on “As the World Turns” in the mid-’80s and again from 1993 to 2008. “It starts when you wake up until nightfall, when sleep is interrupted by a powerful demonic entity threatening to tear your entire life apart.”

The mother-of-three has transformed herself into an investigator, advocate, personal trainer and even amateur psychiatrist for her husband’s cause.

McMahon denies he knowingly worked for the Chinese government, and has long maintained he is a scapegoat used by the Department of Justice to score political points by making an example of him.

Byrne’s continuing fight to exonerate McMahon is told in her new book, “In the Interest of Justice: One Woman’s Fight Against a Weaponized Justice Department to Save Her Husband.”

“You don’t have to have the largest army to conquer the enemy, just the smartest,” she writes.

Armed with her computer and cell phone, Byrne, 55, has become an expert on Operation Fox Hunt — the Chinese government’s scheme to track down and prosecute dissidents in foreign countries, mostly without the cooperation of their sovereign governments.

The Post previously revealed a Chinese police station in Lower Manhattan, where officials allegedly often work with local private investigators to nab Chinese nationals, usually on charges from the motherland.

McMahon was convicted of stalking a New Jersey couple who are Chinese expatriates. He maintains he had no knowledge he had been working on behalf of the PRC.

After his 2020 arrest, McMahon began having panic attacks, according to Byrne, who snapped into action and became his personal trainer.

“You can have one day in bed,” she told her husband. “But ONLY one … Depending on his mood, I balanced the dual role of cheerleader and tough love drill sergeant.”

She also knocked on doors — going to police precincts in New Jersey to tell her husband’s story, and seeking out other private detectives to warn them they were potential DOJ targets and of the need to verify Chinese clients were not working for the PRC.

“You can have one day in bed,” she told her husband. “But ONLY one … Depending on his mood, I balanced the dual role of cheerleader and tough love drill sergeant.”

“Justice would most certainly prevail,” she writes. “We just had to fight.”

Not everyone rushed to help. Byrne went to see 2017 New York City mayoral candidate Bo Dietl, who runs one of the biggest PI firms in the world, based in Manhattan. She claims he listened to her story and then had a colleague hand her a card with a lawyer’s name on it before abruptly ending the meeting.

The Post previously revealed that Dietl went into business with a Chinese security company in 2015 and had boasted on WABC’s “Sid and Friends in the Morning” that he had surveilled the exiled Chinese billionaire dissident Miles Guo. He has never faced any charges related to this and declined to comment Wednesday.

Among those who have backed Bryne’s fight is retired FBI agent Kevin Hecht, one of the foremost experts on Chinese counterintelligence, who began investigating Operation Fox Hunt in 2016.

Despite his extensive experience, Hecht told The Post last week that he was never consulted on McMahon’s case.

He called McMahon’s case an example of “malicious prosecution” on the part of the DOJ.

“The facts of this case show McMahon was not part of the Fox Hunt scheme, but rather, used to obtain documentation of a civil nature, such as real estate and other financial footprints,” Hecht said in a statement.