The 24-year-old was up 19-21, 21-13, 17-12 in Saarbrücken, Germany, when his childhood rival withdrew from the match due to a back injury.
Loh, who's ranked 39th in the world, claimed his first in a Super 500 event of the Badminton World Federation (BWF) World Tour.
That was history in the making as he became the first ever male shuttler from Singapore to win a Superseries/Super 500+ event.
Ongbamrungphan claims Super 500 maiden title
Meanwhile in the women's singles final, Thailand’s Busanan Ongbamrungphan outclassed Singapore's Yeo Jia Min 21-10, 21-14 to win her first Super 500 tournament.
The 25-year-old Thai is ranked 14th in the world, and took 35 minutes to dismiss her 22-year-old opponent.
Doubles action in Germany
The women's doubles final was an all-Japan clash as Hoshi Chisato / Matsuda Aoi defeated their compatriots Iwanaga Rin/Nakanishi Kie in two straight games (22-20, 21-18) after a match with some long and entertaining rallies.
It was an all Indonesian showdown in the men's doubles final with Marcus Fernaldi Gideon/Kevin Sanjaya Sukamuljo triumphant after a fierce 34-minute game against Leo Rolly Carnando/Daniel Marthin with a score of 21-14, 21-19
And in the mixed doubles final it was a battle of the top seeds as Thailand's Dechapol Puavaranukroh/Sapsiree Taerattanachai overcame second seeds Praveen Jordan/Melati Daeva Oktavianti from Indonesia 22-20, 21-14.