The 26-year-old Arsenal Women star was being interviewed at the Pride of Britain Awards, which took place during the Gunners' 1-0 win.
Lionesses skipper Williamson is usually in attendance at the Emirates when free but had to had to head to the event during the blockbuster clash.
Entering the awards on the red carpet, Williamson didn't know Gabriel Martinelli had scored what would be the winning goal.
And she was left stunned when she found out about the Brazilian's strike, which deflected in off Natha Ake.
"They [Arsenal] have scored, it's 1-0," said the reporter.
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"Are you joking?" Williamson replied.
The reporter added: "No, in the 86th minute, Arsenal."
Williamson then turns around in delight, saying: "Did you hear that? Ok, my phone is in my pocket. I am normally at home with the boys."
"Leah, you're 1-0 up," says the journalist. Williamson adds: "Did you see my face then? It is the best news I have heard all day."
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Following the reaction she was instantly hailed by many Arsenal fans, with one fan even dubbing her the 'biggest Gooner'.
Meanwhile, another added: "Protect Leah Williamson at all costs."
And a third said: "Leah Williamson was literally all of us yesterday."
Arsenal boss Mikel Arteta was full of praise for the game-changing substitute.
"His mentality is incredible at his age," Arteta said post-match about the Brazilian.
"It's a joy to have him. Today he could change a game and he was really helpful for us to win the game."
It was the first time the Gunners had beaten the Sky Blues in the Premier League since 2015.
The victory moves Mikel Arteta's side top of the table with north London rivals Tottenham on 20 points.