WASHINGTON — President Trump’s former national security adviser, Michael T. Flynn, pleaded guilty on Friday to lying to the F.B.I. about conversations with the Russian ambassador last December, becoming the first senior White House official to cut a cooperation deal in the special counsel’s wide-ranging inquiry into election interference.

Mr. Flynn’s discussions with Sergey I. Kislyak, the Russian ambassador, were part of a coordinated effort by Mr. Trump’s aides to create foreign policy before they were in power, documents released as part of Mr. Flynn’s plea agreement show. Their efforts undermined the existing policy of President Barack Obama and flouted a warning from a senior Obama administration official to stop meddling in foreign affairs before the inauguration.

The documents do not disclose what Mr. Trump knew about Mr. Flynn’s discussions. But in at least one instance, prosecutors say, Mr. Flynn was directed by a “very senior member” of the presidential transition team to discuss a United Nations resolution. Mr. Trump’s lawyers believe that unnamed aide was Mr. Trump’s son-in-law and close adviser, Jared Kushner, according to a lawyer briefed on the matter.