Brazil's Fonseca came through 2-4, 4-3(8), 4-0, 4-2 to see off his left-handed American opponent.
He is the youngest champion at the event since current world number one Jannik Sinner claimed the title also at the age of 18 five years ago.
"I was really nervous before the match. I knew it was going to be so difficult," Fonseca said before lifting the trophy with tennis legend Rafael Nadal watching on from the stands.
"I played a final against Learner in juniors at the 2023 US Open and I know the way he can play. He is such a nice guy and a great player, so I knew it was going to be difficult, mentally and physically. But I got through."
Fonseca, the lowest-ranked player in the eight-man field, won all five matches he played this week at the Red Sea venue.
He began the year ranked at 730 in the world and having made a maiden ATP quarter-final in Rio this year, he will be aiming to make significant progress in 2025.
"I need to believe when I go before a tournament that I can win," Fonseca said.
"But now I have won it I am thinking, 'Wow, I made it'. I am very proud of myself."