The Wildcats took a 2-0 lead, but never led again after that as Kell quickly took the lead after that, led 8-7 at halftime and stayed in front the rest of the way.
The score was tied 10-all when Sarah Bialecki scored the championship-winning goal for Kell with two minutes left in the contest.
Cabell Zakas scored five goals to lead the way for Westminster. Zakas helped the Wildcats stay in the game as she scored two goals to erase a 9-7 deficit and tie the score at 9-all.
After Kell took a 10-9 lead, Isabella Dadd scored with 5:39 remaining in game to tie it up again at 10-all.
It was the second time that Westminster (17-5) lost to Kell this season, with the Wildcats falling to the Longhorns 14-9 in a regular-season game April 17.
Westminster advanced to the AAAAA-A girls’ championship game with a 19-9 victory over St. Pius X last Tuesday.
The Wildcats also defeated Holy Innocents’ 16-4 in the first round May 6 and Cambridge 17-15 in the quarterfinals May 9.
St. Pius beat Wesleyan 20-6 in the first round May 6 and Pope 13-10 in the quarterfinals to reach the AAAAA-A girls’ final four.
Lovett (18-4) fell just short in its bid for its first state title since 2009 as it lost to Northview 9-8 in the AAAAA-A boys’ finals last Saturday.
The Lions reached the finals after upsetting defending AAAAA-A champion and archrival Westminster 8-5 in the semifinals last Wednesday.
Lovett also defeated Riverwood 17-5 in the first round May 7 and Marist 13-4 in the quarterfinals May 10.
Roswell won the AAAAAA boys’ championship, while Milton took the AAAAAA girls’ title.