The grandson of late New York Giants owner Wellington Mara’s was busted early Tuesday for allegedly setting motorcycles on fire in his Gramercy Park neighborhood, law-enforcement sources told The Post.

Stephen Mara, 26, targeted people making a racket revving their motorcycles and setting off car alarms, the sources said.

Mara — whose dad, Stephen Sr., infamously pummeled a fellow broker on the floor of the New York Stock Exchange in 2006 — was charged with arson.

Sources said a security guard at the Lexington Avenue Armory at East 26th Street, just around the corner from the suspected firebug’s pad, spotted Mara on the street at 2:15 a.m. using a Bic lighter to set fire to the bikes. At least one car also suffered damage.

A Gramercy resident who declined to reveal her name said she was awakened by two booms, and looked out her window to see the guard wrestling with Mara.

“I heard two explosions. I felt the vibrations from them,” the woman said. “I went to my window, and I saw a guy dragging who I thought was a woman, so I called the police.”

Her husband added, “The security guard caught him as he was trying to go back into his building.”

Frank Galea, Mara’s building super, said the bikers are often a nuisance, noting that a rider was revving his engine so much late one night last week, it set off his carbon-monoxide alarm.

“I went outside, and the guy told me to go f–k myself,” he said. Still, Galea added, Mara was acting like a maniac. “For someone to do this, you have to be sick in the head,’’ he said.

The suspected firebug was charged with three counts of arson Tuesday night at Manhattan Criminal Court and released on $40,000 bail.

Mara is an executive for Major League Baseball, a coordinator of on-field operations, a gig he landed before the 2012 season.

Ironically, his duties include “assisting with disciplinary action,” according to MLB.

He is a cousin of “The Girl With the Dragon Tattoo’’ star Rooney Mara. Their fathers are brothers.

A woman who answered the phone at his parents’ Westchester County home issued a terse “no comment” and hung up.

Mara’s dad got into a fight with a Philadelphia Eagles fan-broker on the floor of the NYSE seven years ago.

The fan taunted him because the Eagles had defeated the Giants 36-22. The older Mara was suspended by the NYSE for two weeks.

Additional reporting by Jennifer Bain and Amber Sutherland