Roby, who was acquired in a draft-night deal with the Detroit Pistons on June 26, was selected 45th overall.
The Mavs also received two future second-round picks in the deal that sent Detroit the draft rights to 37th overall pick Deividas Sirvydis.
Roby (6-foot-8, 230 pounds) played three seasons at Nebraska. As a junior, he started all 35 games, averaging 11.8 points, 6.9 rebounds, 1.9 assists and 1.3 steals
He shot .476 from the field and .702 from the free-throw line in his college career.
He started all five games for the Mavs in the Summer League in July, averaging 8.8 points, a team-high 5.2 rebounds, 1.4 assists and a team-high 1.2 blocks.