In the final at 6 p.m. Saturday at Villanova, the eighth-seeded Red Raiders will play Villa Maria, which beat Pope John Paul II, 52-33, in the earlier game at Wissahickon.
"I was really nervous," said Williamson, who played through a sore shoulder to lead her team with 19 points, including a free throw that gave Radnor a one-point lead with less than 15 seconds remaining.
"But it's just a foul shot. I can do this. I just zoned everybody out and made it."
In the other semifinal, Mora Fetterolf of third-seeded Villa Maria had the ball with a big lead and her teammates and fans shouting a countdown until the final buzzer, so she turned and launched a three in excitement just before time ran out.
As with most of Villa Maria's night, it worked out, and the 6-foot interior player who "never shoots" drained the three to close out the win against second-seeded Pope John Paul II (18-5).
"I don't think she's shot a three all season," said Molly Namiotka, who led Villa Maria with 19 points. "That was the highlight of the year."
"I was just excited we were winning," said Fetterolf, who walked back to her bench to a barrage of teammates imitating her nonchalant release. "Everyone was just surprised I took it because I don't usually shoot the ball."
Villa Maria (15-10) didn't have the regular season it expected, but has found new life in pursuit of its fourth consecutive district title.
"We knew that we weren't going to let up," said Fetterolf, who had seven of her 11 points in the fourth quarter. "We're seniors, and since freshman year, we've gone to the championship game and we've won.
"We want to win the district championship again. . . . It's like a brand-new season. It's a clean slate for us."
Pope John Paul II 10 5 10 8 - 33
Villa Maria 8 10 14 20 - 52
PJP: Lauren Dao 6, Stephanie Petery 6, Gabby Troisi 13, Jenna Widdicombe 2, Dahlia Wilson 6.
VM: Jackie Carlin 14, Mora Fetterolf 11, Danielle Knopp 1, Katie Mooney 4, Molly Namiotka 19, Devon Tirney 3.
Merion Mercy 11 7 8 14 - 40
Radnor 13 8 9 13 - 43
MM: Caroline Corzel 12, Tina DiGiacomo 4, Gabby Heinsinger 2, Valez Jackson 18, Mariah Quinn 4.
R: Caitlin Connor 2, Kristin Hamill 8, Nicole Massimino 4, Lauren Pacifico 10, Myonie Williamson 19.