In 2009–10, Mercer played in all 33 games for the Bulls, starting 30 of them. He ranked fifth on the team with 9.2 points per game while also adding 4.5 rebounds in 28.6 minutes per contest. He also finished third on the team with 48 assists and second with 44 steals. On January 5, 2010, he had his first career double-double with 10 points and a career-high 10 rebounds against Notre Dame. During the season, he had two 19-point games, a season high.