The 26-year-old recovered from a set down on Court Philippe Chatrier to finally end her wait for major glory, coming out on top 3-6 6-4 6-1 in Paris.
Halep, beaten in three previous Open finals, lost the only break of serve in the opening set four games in, which Stephens was able to build on.
After racing a set ahead, the reigning US Open champion then broke her opponent in the opening game of a pivotal second set that swung one way and the other.
Despite a lack of success to build on in such situations, Halep dug deep and took four games in a row on her way to levelling up the match at 1-1.
The third set would prove to be a little more straightforward for the Romanian, as she raced away by holding her serve throughout and took the first match point when it arrived to get off the mark with her first Open triumph.