SEOUL—Before reporting for his mandatory military service, Kim Sang-heon thought skin care didn’t require anything more than a bar of soap.

His views have changed since a two-year tour in South Korea’s army.

“I try to use a facial pack every night,” says Mr. Kim, 28 years old, who now works at an office in Seoul. “It’s been a daily routine since my time in the military.”

South Korea is a force in the global beauty industry and its carefully groomed pop idols have become global stars. And for many guys here, the gateway to face serums and 10-step skin-care regimens is their time as military conscripts.

During his service, Mr. Kim developed a routine while posted at an observation post along the barbed-wire fences of the inter-Korean demilitarized zone.