The youngest Kardashian family member is making her fortune from her best-selling cosmetics business.
The 21-year-old founded and owns Kylie Cosmetics, the three-year-old beauty business that generated an estimated $360m in sales last year.
She reached the milestone earlier than Facebook founder Mark Zuckerberg who became a billionaire aged 23.
"I didn't expect anything. I did not foresee the future.
"But [the recognition] feels really good. That's a nice pat on the back," Ms Jenner told Forbes.
The list shows Amazon's founder, Jeff Bezos, remains the world's richest man.
His fortune totals $131bn, according to Forbes, up $19bn from 2018.
But the billionaires' combined worth is down from $9.1 trillion at $8.7tn.
Facebook founder Mark Zuckerberg's wealth is among those falling.
It has dropped by $8.7bn (£6.6bn) in the past year to $62.3bn, according to the Forbes list.