Arnold played a central role in the penalty drama on Saturday which sent co-hosts Australia into the last four of the World Cup for the first time.
She saved three penalty kicks -- and missed one of her own -- to help propel the Matildas to a thrilling 7-6 shootout win over France in the quarter-finals.
It has been a whirlwind few months for the 29-year-old.
The West Ham United stopper has been part of the Matildas set-up for more than a decade but she was third choice for much of the time, spending the 2015 and 2019 World Cups on the bench.
It is only this year that Arnold finally became coach Tony Gustavsson's first-choice between the posts.
The past few months have been significant in another way for Arnold off the pitch.
In April she posted a video on Instagram of herself collecting some hearing aids.
"Those closest to me know how long I've avoided this day, but here's a little glimpse of a life-changing day for me," she wrote.
Arnold does not wear hearing aids during games because she would be "fiddling with them too much" and has said that not wearing them "is still quite normal for me, doesn't affect me too much".
But she told Optus Sport ahead of the World Cup: "It's still a whole different world when I put my hearing aids in.
"It's like, 'What have I been missing out on for 25 years?'"
- 'A cult hero' -
Cortnee Vine held her nerve to score the winning penalty in front of 50,000 in Brisbane on Saturday and set up a date with European champions England in Sydney.
But it was Arnold who scooped the player of the match award and shared the headlines with Vine in Australia, where the team's exploits are now daily front- and back-page news.
National broadcaster ABC called Arnold "a cult hero" and The Sydney Morning Herald said she was "a national hero".
Reflecting her sudden rise to national acclaim, the newspaper ran a story headlined, "Who is Mackenzie Arnold? Meet the goalkeeper with nerves of steel who sent the Matildas to the semi-finals."
The Australian players sang Arnold's name in the dressing room afterwards and vice-skipper Steph Catley said: "She's incredible. She did something so special tonight."
Arnold started her career in Australian women's football, before a move to Norway in 2018.
There was an unsuccessful stint at Chicago Red Stars, followed by the move to West Ham in 2020.
Her spot-kick may have crashed off a post against France, but it was not a surprise to see her take a penalty -- she is adept with her feet, once coming on as midfielder for West Ham in an FA Cup game.
Arnold came to the World Cup off the back of a successful season in the Women's Super League, making more saves than any other goalkeeper in the WSL.
- England warning -
Following her display on Saturday, which included keeping France scoreless over 120 minutes to force the penalty shootout, Arnold said she was not even feeling 100 percent.
Looking a little glassy-eyed, she told reporters who swarmed around her: "I haven't been too well in the last couple of weeks.
"I'm still getting over that -- hence the eyes. This isn't tears yet."
Arnold put her penalty-saving heroics down to instinct more than anything else.
"We do have a little bit of research obviously that goes on behind the scenes, and communication with coaches as well," she said.
"I think I just tend to stay a little bit calm, but I wouldn't say I read it too well. It's gut instinct that I just go with."
It has worked for Arnold in the past -- she once saved two penalties in one game against Manchester City.
England have been warned.
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