She defeated former world Number one, Venezuelan-Spanish Garbine Muguruza in three sets 4-6 6-2 6-2, breaking Garbine twice in the final set to clinch a historic Grand Slam.
Garbine Muguruza won the French Open in 2016 and Wimbledon in 2017, but failed to add to her major titles after falling to the American.
The unheralded American seeded 14th gained attention after she beat teenager Coco Gauff in the 4th round. She then went on to knock out Australian title hope Ashleigh Barty in the semi-final.
The 14th seed, who will now jump as high as seventh in the world and usurp Serena Williams as America’s number one, was in tears at the end and headed straight for her father Alexander, who is her coach.
It was the final twist in a tournament of upsets, after Williams went out in the third round and Kenin upset Australia’s world number one Ashleigh Barty in the semi-finals.
“My dream officially came true, I can’t describe this feeling,” said Moscow-born Kenin. “It’s amazing, dreams come true. If you have dreams, go for it, it’s going to come true.”
Spain’s Muguruza, 26, was unseeded for the first time at a Slam since 2014 having suffered a marked loss of form in the last 18 months.
She was resurgent in Melbourne over the past fortnight, but after grabbing the first-set lead, her serve failed her spectacularly.
She totted up eight double-faults in all, three of them in the final game — including one on the second championship point, handing the title to her younger opponent.
Muguruza drew first blood at Rod Laver Arena, where the roof was closed for rain in Melbourne, getting the first break of serve.
Moscow-born Kenin, who ended the fairytale run of 15-year-old Coco Gauff on her way to the final, bounced her American stars-and-stripes racquet on the hardcourt in irritation.
The 2016 French Open and 2017 Wimbledon winner Muguruza took the first set in 52 minutes when the young American planted her forehand out.
But the aggressive Kenin upped the ante in the second set, breaking her more experienced opponent in the fourth game and easily holding to sprint into a 4-1 lead.
Kenin, who won their only previous encounter in three sets, grabbed the second set in an emphatic 32 minutes. A rattled Muguruza was seen briefly by a physio for what appeared to be a lower-back problem.
Into the deciding set and the gutsy Kenin saved three game points in a pivotal fifth game, tossing the ball back over her head by way of celebration.
Kenin, who was overshadowed in the build-up to Melbourne by the likes of Williams, Osaka and Barty, adds by far the biggest title of her fast-burgeoning career to the three WTA crowns she won last year.
“The past two weeks have been the best of my life,” she told Rod Laver Arena, as her father — who left the Soviet Union in 1987 to give the family a better life — filmed her victory speech on his phone.
“I love you guys from the bottom of my heart, thank you so much,” she told the crowd.
At 21 years and 80 days, Kenin is 22 days younger than Japan’s Naomi Osaka when she won the title last year.
Kenin is the youngest Australian Open champion since Maria Sharapova — her idol — won aged 20 in 2008.