Henderson, whose major breakthrough came at the 2016 Women's PGA Championship, holed a 10-foot birdie putt for a 17-under total on the last - where just moments earlier Sophia Schubert had missed from a similar distance to get to the same mark.
It gave the 24-year-old, who had begun the day with a two-stroke advantage, victory by one from the American rookie after a mixed round saw her eventually sign for a level-par 71 having been two over for her front nine.
Kettering's Hull had started six shots adrift but a second successive 67 kept her in contention until the later stages and was good enough for a share of third place.
"It was definitely an interesting day. Not the day I wanted but I stayed patient," the 24-year-old Henderson said in her post-round interview.
"The saying is majors are won on the back nine on Sunday so I kept that in mind.
"I'm super-excited to have my second major win."