Forward Luis Cartagena opened up the scoring in the 15th minute, and it looked like United would hold on to the 1-0 lead until Dallas equalized on a goal by Jonathan Top in the 75th minute.
United prevailed, 6-5, on spot kicks.
"We are very proud to have won the Cup for the second time in three years," said United Director of Youth Development John Maessner. "Our boys showed tremendous heart, passion and team unity, and demonstrated that we are the best U-17 team in Major League Soccer. We have a very talented, very young group of players with an extremely bright future. It was wonderful to watch our young boys play with confidence and heart as a well organized, disciplined unit both on and off the field. They defended well, attacked with numbers and speed and created many scoring chances throughout the week. My hat goes off to our staff as well."
A goal by Cartagena gave United a 1-0 victory over the Kansas City Wizards in the decisive group match.