Romo threw a scoring 30-yard scoring pass to Patrick Crayton midway through the second quarter; and Mike Vanderjagt kicked two field goals to give Dallas its halftime lead after Neil Rakers kicked a field goal for the Cardinals.
Romo then tossed a 51-yard touchdown pass to Terrell Owens in the third quarter to make the Cowboys lead, 20-3.
Marion Barber scored on a 1-yard run in the fourth quarter to increase the Cowboys advantage to 27-3.
Dallas (5-4) is tied with Philadelphia for second place in the NFC East.
Matt Leinart passed for 216 yards and two interceptions but rushed for a fourth-quarter touchdown; and Edgerrin James gained 68 yards on 15 carries for the Cardinals (1-8), whose losing streak grew to eight games.