Trying to hold off rookie and former baseball player Drew Henson for the job to back up the 40-year-old Vinny Testaverde, Romo was the favorite entering Thursday's contest.
Romo all but cemented the job as he went 12-of-14 for 141 yards, including a 15-yard touchdown pass to Antonio Bryant in the third quarter.
"I've been gaining confidence each week," said Romo. "I was able to get into a rhythm tonight and our wide receivers and offensive line did great work tonight."
Henson came on late in the third quarter, failing to complete any of his four attempts while throwing an interception.
Eddie George and ReShard Lee added touchdown runs for Dallas (3-1), which opens the regular season at Minnesota on September
"I like the players. I like coaching the team, but we have a lot of work to do," said head coach Bill Parcells afterwards. "Anybody watching us play can tell that. The practice games are over. It's time to get ready for Minnesota."
The Cowboys were playing for the second time in four days after beating Tennessee on Monday, and some of their veterans showed it.
Starting quarterback Vinny Testaverde completed just 4-of-10 passes and also threw and interception, and running back Eddie George rushed for just eight total yards on nine carries.
George finished the preseason with 51 yards on 24 carries.
"I have to relax as much as possible and take what the defense gives me," George said. "I'll continue to work hard, look at my mistakes and move on."
Trent Green and Todd Collins each passed for a touchdown for Kansas City (1-3), which opens the season at Denver on September 12.
The Cowboys (3-1) finished the preseason with a winning record for only the third time since Jones bought the team in 1989.
Romo Shines in Cowboys Win
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