Mahut, who was the 2007 runner-up at the grass-court event in Newport, R.I., overcame Hewitt 5-7, 7-5, 6-3 for his second career title.
The 31-year-old logged his first-ever win last month at the Topshelf Open and improved to 2-2 all-time in finals with Sunday's triumph.
It came shortly after Mahut had dispatched Michael Russell 6-2, 6-2 in a semifinal that had been pushed back after Saturday's scheduled date was rained out.
The fourth-seeded Hewitt had reached the title tilt after defeating two-time defending champ John Isner in Sunday's other semifinal.
Hewitt, a former world No. 1, fell to 28-17 in ATP World Tour finals.