Former Nassau County Executive Ed Mangano treated US Sen. Charles Schumer to a free lunch — because Mangano knew he wouldn’t get a bill, the restaurant’s crooked owner testified today.

Harendra Singh, the prosecution’s star witness at Mangano’s corruption trial, said Mangano brought Schumer to an eatery he described as his “East Meadow location” following an unspecified event in September 2010.

When asked by defense lawyer Kevin Keating if the visit by Schumer — who’s now the Senate minority leader — would “make you happy,” Singh answered, “No.”

Pressed for the reason, Singh said he thought the main reason Mangano was bringing Schumer to his restaurant was because “he knew it wouldn’t cost him anything.”

Mangano is accused of accepted hundreds of thousands of dollars worth of bribes from Singh — including a no-show job for his wife, Linda, who is a co-defendant — in exchange for official favors.

Former Singh employees have told Newsday that they were ordered to let Mangano and other officials eat and drink for free at his since-shuttered restaurants, where the pols racked up bills that would have reached several hundred dollars.

Mangano was a big fan of the omelets at Singh’s Chow Down Diner in East Meadow and the slices at his BeSi Pizzeria in Bethpage, and also frequently dined at the HR Singletons Steak House in Bethpage, the Woodlands catering hall at the Oyster Bay golf course and the Water’s Edge restaurant in Queens, the workers said.

Singh also claims that he bribed Mayor Bill de Blasio with campaign cash for favors that included a lease extension on Water’s Edge, but prosecutors last year declined to charge de Blasio, who has vehemently denied the bribery allegations.