The Dolphins took a 6-0 lead on a pair of Dan Carpenter field goals and did not score again until the Cowboys had built a 27-6 advantage.
Dallas rested most of its starters while finishing its exhibition season at 3-1. Quarterback Stephen McGee, likely needing a solid performance to make the roster, went 9-of-18 for 124 yards.
The Cowboys will open the regular season next Wednesday on the road against the New York Giants in the NFL's kickoff contest.
Miami went winless in preseason for the first time in 23 years. Rookie Ryan Tannehill, who has been announced as the regular-season starter, played two series and went 5-of-7 for 35 yards. The Dolphins will open Sept. 9 in Houston.