WASHINGTON—Roger Stone, a flamboyant Republican operative and longtime adviser to President Trump, was found guilty Friday of lying to Congress and witness tampering, making him the latest member of the president’s circle to be convicted on federal charges.

Mr. Stone was found guilty on all charges: five counts of false statements, one count of witness tampering and one count of obstruction of a congressional proceeding. In a trial lasting just over a week, prosecutors made the case that he engaged in an effort to mislead...