Matildas striker and league golden boot winner McNamara, 23, opened the scoring with a world-class long-range strike from the corner of the box in the 41st minute and added a second in the 43rd at AAMI Park on Saturday.
Fellow Matilda Leticia McKenna scored a terrific long-range goal of her own in the 49th minute, before Phoenix striker Makala Woods scored in the 52nd to give the strong Wellington fan contingent something to cheer about.
With her heroics, McNamara delivered City a premiership-championship double in front of 7174 fans and their first grand final win since the 2019-20 season, while denying Bev Priestman's Phoenix their first title.
McNamara, who has come back from three ACL tears to become the league's best striker, was sidelined when City lost to Sydney FC in the 2023-24 decider.
"This morning I actually realised it was my first grand final," McNamara said.
"Obviously, watching the last one was kind of heartbreaking. I remember shedding a few tears after that loss, because you kind of feel helpless.
"I just wanted to put my best foot forward today in probably one of our biggest games of the year and just so happy that we won.
"There was a bit of pressure to score, probably a lot of it was internal, knowing that I haven't scored for a few games. So I was just super happy to get that in the back of the net."
McNamara also paid tribute to former coach Rado Vidosic, who died in January.
"That was kind of for him today," she said.
Michael Matricianni's City draw level with Sydney FC on five championships and 10 ALW trophies overall.
City can yet claim a treble if they triumph in the Women's Asian Champions League.
They will stay in an airport hotel on Saturday night then depart for Suwon early on Sunday for Wednesday's semi-final against Japanese club Tokyo Verdy Beleza.
"We know the job's not done yet," skipper Rebekah Stott said.
From the first whistle City were on top with Shelby McMahon and McKenna running the show and McNamara, who went goalless in her previous five games, was a constant threat.
For the opener, Danielle Turner curled a super ball towards McNamara and the striker worked the ball to Aideen Keane, then received it back and beat Victoria Esson with a sensational curling strike.
"There was so much space around me that I thought, why not?" McNamara said.
Two minutes later, McMahon escaped multiple players then brilliantly released McNamara down the right and the City striker burst forward to fire home her second.
"I was laughing at some of the comments that I was reading online, that 'oh, she hasn't scored in five games'," Matricciani said.
"I'm going 'great, because when she hasn't scored, it's even scarier'.
"So she proved she should be proud of herself first and foremost, but yeah, maybe she shut up a few people as well."
McKenna snagged City's third when she was given too much space on the edge of the box and curled the ball off the post into the top corner.
Wellington snatched one back when Mackenzie Anthony forced a save from Mieres, but Woods tapped home the rebound.
"We didn't give the best account of ourselves in the first half and when you're not at your best, a team like City can punish you, right?" Priestman said.
"... What I am proud of is the second half we give a much better account of ourselves, we never gave up, which I think is important."
