The second-seeded Thunderbirds had a chance to tie the game in the dying seconds of the extra session but Kris Young's contested shot came up short. Young had a chance to win the game late in regulation but Marika Guerin stole the ball as the UBC star drove to the basket with a few seconds left to play.
McGill will play the top-seeded Windsor Lancers in Sunday's final after Windsor downed the Saskatchewan Huskies 75-61 in the other semi. UBC will play Saskatchewan for the bronze.
Dianna Ros had 12 points for the Martlets. McGill became the first Quebec team to reach the national final since Laval in 2002.
"It was exciting, we were playing against a really disciplined team," said McGill head coach Ryan Thorne. "It was like a heavyweight boxing match. We got up, they fought back, then they got up, we fought back. This is what happens when there are two really good teams out there.
"We fell asleep a few times and when you do that, a team well-coached and executing as good as UBC will make you pay for it. That's what kind of got us into that overtime situation."
UBC's Harleen Sidhu led all scorers with 20 points.
"It was a real tough, physical game for 45 minutes. It was a great, quality match for a semifinal," said Thunderbirds coach Deb Huband. "We struggled offensively, we turned the ball over twice as much as we normally do. We were poor from the free-throw line and we had a little bit of an uncharacteristic performance out there. McGill is very deserving of the win."
In the late semifinal game Saturday, Korissa Williams poured in 24 points and grabbed 12 rebounds as the Lancers took a step toward claiming their fifth straight national title.
Cheyanne Roger had 16 points and seven boards for Windsor and Kristine Lalonde added 12 more points while sinking 4-of-5 three-pointers.
Laura Dally led Saskatchewan with 22 points. Riley Humbert chipped in with 10 and Desarae Hogberg had eight points and seven rebounds off the bench.
Windsor led 20-8 after one quarter, 48-26 at the half and 64-43 after three. The Huskies slowly chipped away at the deficit, outscoring the Lancers 18-11 in the fourth, but couldn't complete the comeback.
In the fifth-place game Saturday, the Alberta Pandas topped the Ryerson Rams 73-65.