A former child actor from Brooklyn who once played a kiddie gangster netted a half-million dollars in a kickback scheme worthy of the real Mafia, authorities said Tuesday.

John Cassisi — who played baby-faced gangland boss Fat Sam in the 1976 flick “Bugsy Malone” starring Scott Baio and Jodie Foster — pulled off his illicit scheme while working in a top construction-finance post at Citibank, sources said.

Cassisi was director of global construction for Citibank Realty Services, making him responsible for overseeing the bank’s construction projects and ultimately the awarding of lucrative contracts.

The 53-year-old Bensonhurst-bred tough guy accepted cash, lavish trips and free work on his home in East Northport, LI, in exchange for steering contracts to certain firms.

One electrical company that benefited rewarded him with a $21,500 trip to Alaska.

Cassisi — who also appeared in the 1977 TV series “Fish’’ with Abe Vigoda and the 1982 movie “Gemini’’ with Danny Aiello — pleaded guilty in Manhattan Supreme Court on Tuesday to money-laundering and bribery.

He was given two to six years in prison on the money-laundering rap and another one to three for the bribery, but the sentences will run concurrently.

The former actor was immediately taken into custody after sentencing.

“Bugsy Malone’’ director Alan Parker last month recalled casting the unknown Cassisi and his young cohorts for the song-and-dance flick, which was supposed to be a comic nod to “The Godfather.’’

“We recorded almost 100 school Christmas shows and saw almost 10,000 kids’’ looking for just the right actors, Parker told the British newspaper The Guardian.

“In a Catholic school in Brooklyn, I asked: “Who’s the most badly behaved kid in the class?” Thirty kids pointed to one chubby little boy at the back. ‘Cassisi!’ they screamed. John Cassisi put his hands up and smiled. Although he had never acted before, he yelled out lines from the script with gusto. I knew right away we had our Fat Sam.”

Cassisi’s lawyer told the judge Tuesday that “Mr. Cassisi has accepted responsibility’’ and “wants to move on with his life and serve his sentence so he can rejoin his family.”

His IMDb Web-page biography states that he is married with three children.

Citibank said in a statement that it was “appalled’’ by his actions and those of another employee snared in the scheme.

The other worker, Arthur Fazio, was Cassisi’s underling and faked work orders to cover up the illicit construction on Cassisi’s home.

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