The 23-year-old got by the fourth seed and 2014 French Open finalist in two sets, winning 6-3 6-3.
Mladenovic came to California after a breakthrough month that saw her claim her maiden WTA title at the St. Petersburg Ladies Trophy and reach another final at the Abierto Mexicano Telcel.
Halep, however, has not played a great deal of tennis lately because of a left knee injury, and whilst she showed signs of her best, the Romanian proved too rusty against Mladenovic, who required 1-hour 42-minutes to earn her second victory over a player in the top five in two months.
"We always have very tough battles," said Mladenovic.
"I was actually up in our head-to-head, 2-1. That's not very often against a Top 5 player!
"I think that match was a big win for me, physically. I was there solid and hanging in there and putting sometimes balls back and being brave. I perfectly played the game plan I had. Solid at times, make her play balls and making her take some risk at the time. Sometimes when I had the shorter one, like, stepping in it, playing my heavy shots with the forehand, and I think that worked pretty well."
Mladenovic will face American Lauren Davis in round four.