An MTA board member went on a tirade against CEO Janno Lieber last month that ended with him giving the chairman the finger — all over the loss of his authority-issued parking permit, sources said.

Nassau County rep David S. Mack lost his parking placard privileges in June after a top-level MTA exec was caught using an expired permit to park near MTA HQ.

Mack, 81, is known to fall asleep during board proceedings — but was lit up on June 29 when he blew past Lieber’s secretary and barged into an early-morning meeting demanding to speak with the chairman alone, according to a source who witnessed the interaction.

“He’s like waving an ID or some PBA card, saying, ‘I am law enforcement!’,” the witness said. “It was like a kid having a tantrum.”

Mack “lost it” and began making threats — including what observers believe was an “inappropriate hand gesture,” the source said.

“He concluded the conversation making warnings that he was going to cause problems,” the source said. “He was like, ‘I know people.'”

After being told in no uncertain terms he would not receive a placard, the longtime Long Island power player shifted to blasting Lieber for not installing his preferred pick for MTA Police Chief.

“He started going on about how… he had a candidate for police chief that should have been hired and Janno should have overruled to the hiring committee,” the source said.

“It was so old timey. Janno was like, ‘Listen there’s a process,’ and Mack was like, ‘Just overrule him! Take my guy! What’s wrong with you?'”

Mack has been known to doze off during MTA meetings, and sources said his board colleagues have discussed taking responsibility for keeping him awake.

He spoke more times at a meeting immediately following his tirade than he had in several years, according to longtime observers.

“It’s like he woke up from his slumber — not because he has interest in transportation and the betterment of the region, but because he had a parking placard,” one source said.

Mack’s comments included expressing his opposition to Metro-North President Cathy Rinaldi simultaneously leading the Long Island Railroad, as she has since February.

“It was unusual to see Mack so engaged given that he rarely speaks up during meetings about these topics,” another insider said.

An MTA official said placard holders were required to “justify” their having a permit as part of this year’s renewal process. Placards must be used “for official business, responding to an emergency” or by “a member of law enforcement,” the official said.

“The MTAPD issues placards to police and others who need to respond to incidents at all hours and locations for official business,” MTA spokesman Tim Minton said in a statement.

“Board Member Mack as yet has provided no basis in making his request for a policy exception, which therefore was denied.”

Mack could not be reached for comment. He told Newsday he uses his placard to drive to and park at MTA board meetings, which are held in Lower Manhattan.

The real estate developer and Long Island political big-wig has represented Nassau County on the MTA board since 2018.

A previous stint on board from 1993 to 2009 ended amid an investigation into the State Police’s decision to make him a deputy superintendent — complete with badge and uniform — despite his having no police experience and no actual responsibilities.

He and his brother Bill also took heat in 2021 for arranging for wealthy friends in Manhattan and Palm Beach to skip the line for COVID-19 vaccines.