Johnson headed home a wonderful cross from Charli Grant in the 38th minute in front of 43,020 fans at Marvel Stadium.
Kaitlyn Torpey added a second when she attempted to launch a cross into the area but shanked it and watched on in disbelief as it nestled in the top corner.
"It was pretty surreal," she said.
"Until that moment, I was kind of just focused on doing my job. I was just like running, get back post, watching the cross from Charli and when it hit the back of the net, it was pretty cool.
"Obviously, my celebration. I don't think I believed what happened and I still don't think I have."
Replays showed the attacker was offside but there is no VAR in friendlies and Johnson won't care.
"I mean, if it doesn't get called, is it offside?" she quipped.
Johnson, who had scored three goals in six games for Calgary Wild in Canada's Northern Super League, was among Australia's best, impressing with her deft footwork and strength on the ball.
Australia doubled their lead in the 69th minute when Kaitlyn Torpey shanked a cross into the top corner.
Interim boss Sermanni, who has coached the Matildas across three stints, appears likely to finish up after Monday night's second game in Canberra.
"I've been extraordinarily lucky. I've had three spells with the team," he said.
"And if you could pick three spells to have, I would have picked the three that I had."
Joe Montemurro is widely expected to take on the full-time job after the current international window.
Sermanni has consistently debuted players across his three tenures and relished in Johnson, 21, being the latest success story.
"It still feels as good as ever," he said.
"It feels great when see a young player coming into the team and you can get them on the field, and particularly, then get them on the field and start them and it makes it even better when you see how well Kahli played tonight."
In rare post-match scenes after a friendly, Argentina coach German Portanova teed off on the referees, indicating Australia had received the rub of the green from Thai official Pansa Chaisanit.
"The first goal is offside," Portanova said through an interpreter.
"I know that this was a celebration of the Matildas. But we came here to compete, and we came here looking ahead to the Copa America and wanting to do our best.
"But believe me when I say, when it was one against the other, (calls were made) in favour of Australia.
"Trying to play with this style of refereeing does make me angry, very angry - when we just see these fouls against us over and over throughout the game, the referee highlighting things that our team has done that just simply haven't happened.
"When these are happening and we see once again awarded to the other team, this is where we tend to lose confidence and we're not able to play our game. I honestly think that with a fairer referee, we would be there."
Sermanni dismissed the suggestion things had gone Australia's way, bar the offside goal.
The Matildas' starting line-up had just 438 combined caps heading into the game - with Emily van Egmond and Alanna Kennedy claiming 293 of them.
Captain Steph Catley and Arsenal teammates Caitlin Foord and Kyra Cooney-Cross didn't play after arriving in camp on Wednesday night following their Women's Champions League triumph.
Sam Kerr, Ellie Carpenter, Hayley Raso, Katrina Gorry and Mackenzie Arnold aren't in the current squad.
But Australia still had 16 shots (six on target) to six, while Argentina didn't register a shot on target until the 80th minute.
