The leader of the Nassau County Police Department’s 2,000-member patrol division is retiring after more than three decades in law enforcement, officials said Tuesday.

Chief of Patrol Frank Kirby, who spent 31 years as a Nassau cop and worked previously for three years at the NYPD, is retiring from the police department in March, he confirmed Tuesday.

“Now is a good time,” said Kirby, 57, in a brief interview Tuesday. “It’s always sad to leave. It’s been a good career.”

Kirby, who assumed the job of chief of patrol in November 2010, was described by colleagues not authorized to speak publicly as “detail-oriented” and “calm, steady and a straight shooter” who had “tremendous knowledge” of his command.

“The patrol division is stocked with good cops; they do a good job every single day,” Kirby said. “Overtime’s down, summons activity is up, arrests are up. . . . Everything that you could measure is good.”

Acting Police Commissioner Thomas Krumpter in a statement said the chief “has served the NCPD in an admirable and outstanding manner for over 30 years. . . . His dedication to the department and the residents of Nassau County is second to none and he will be greatly missed.”

Kirby, who received numerous department awards over his career, most recently was the deputy incident commander for the presidential debate at Hofstra University in September and the incident commander for the debate in 2012.

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Chief of Department Steven Skrynecki, who himself is leaving Nassau early next year to be the chief of police in Southampton Town, said Kirby showed a keen ability to organize security for large-scale events “to provide maximum public safety while managing resources.”

Kirby was “among the most dedicated and talented individuals I have worked with,” Skrynecki said, adding that “on a personal level I consider Chief Kirby a valued friend as well as a colleague and wish him the very best in his future endeavours.”

Kirby, who said he had no specific plans for retirement, said: “I feel I’ve accomplished a lot. And I did it on my own. So that’s what I’m most proud of. I had nobody looking out for me, but me. . . . I got to where I am by working hard.”