The questioning was brief - 90 minutes or less - and tightly focused on Flynn. It was in part aimed at determining whether Kushner had any exculpatory information on Flynn, according to a person familiar with Mueller's investigation.
Kushner and Flynn were both prominent figures in the Trump campaign, the presidential transition and the early days of the Trump administration.
The two also took part in discussions during the presidential transition with Sergey Kislyak, Russia's ambassador to the United States at the time, about establishing a backchannel between the two countries, a possible indication of prosecutors' interest given Mueller's mandate to probe contacts between Trump associates and the Kremlin.
Flynn was forced to resign from the White House in February after officials concluded that he had misled them about his contacts with Kislyak during the transition period.
Weeks before he was fired, he was interviewed by the FBI about that communication, and former FBI Director James Comey has said Flynn was under investigation for potentially lying to federal agents about the nature of their conversation.
Mueller's grand jury had planned in coming days to hear testimony from an employee of a public relations company that worked with Flynn's firm on $530,000 worth of lobbying and investigative research for a Turkish businessman.
The testimony was slated to focus on Flynn's firm's interactions with congressional staff. But it was abruptly postponed by prosecutors.
The details of Kushner's questioning and the postponement of the grand jury testimony were confirmed by people familiar with Mueller's investigation.
They spoke on condition of anonymity because they weren't authorized to publicly discuss the ongoing investigation.
Asked about the meeting with Mueller, Kushner's attorney, Abbe Lowell, did not elaborate on the nature of the question, saying only in a statement his client 'has voluntarily cooperated with all relevant inquiries and will continue to do so.'
Mueller's spokesman, Peter Carr, has yet to comment on the special counsel's ongoing investigation that has now stretched into its seventh month.
Instead, the special counsel has decided to speak in indictments and plea agreements. In the meantime, Washington - and the country - wait for the next one