Stephens, who was ranked 957 in the world in July after undergoing foot surgery earlier in the year, came through in straight sets – 6-3, 6-0.
She overpowered Venus Williams in her semi-final, and many expected a tight battle in the final.
Stephens refused to be bullied by Keys, who was unable to exert pressure on her opponent.
After clinching the first set, the momentum was with Stephens.
She was able to break early on in the second set, and from there in, there was no way back.
Stephens continued to build up steam and she wrapped up in the match in little over an hour.
She has now become the fourth different winner of the Grand Slam tournaments this year.
With victory, she has now broken into the world’s top 20.
Speaking after the match, Stephens joked: “I should just retire now.
“I told Maddy, I’m never going to be able to top this.”
With her win, Stephens collected a cheque for £2.8million.