In addition to his speaking and teaching roles, Mueller also joined the law firm WilmerHale as a partner in its Washington office in 2014. Among other roles at the firm, he oversaw the independent investigation into the NFL's conduct surrounding the video that appeared to show NFL player Ray Rice assaulting his fiancée. In January 2016, he was appointed as Settlement Master in the U.S. consumer litigation over the Volkswagen emissions scandal; as of May 11, 2017, the scandal has resulted in $11.2billion in customer settlements.