McMaster's signature capped a seven-year effort to close the so-called liquor-cycle loophole, though it won't take effect for 18 months.
His predecessor, Nikki Haley, vetoed a similar measure last year, calling it government overreach to require reflective vests for nighttime driving and helmets for drivers under 21. The compromise approved by legislators last week removed the vest requirement.
"There are going to be lives saved," said Sen. Greg Hembree, a Republican from North Myrtle Beach.
The former prosecutor has been pushing for moped safety legislation since his 2012 election, calling mopeds the most dangerous vehicles for people to drive, partly due to the lack of regulations.
According to the state Department of Public Safety, 41 moped drivers were killed last year in 785 crashes. More than 80 percent resulted in injuries.
Mopeds are popular in South Carolina among tourists, college students and drivers who lose their license.
Drivers "can be stinking drunk on a moped and can't be arrested," Hembree has said.
That's because state law specifically excluded mopeds from the definition of a motor vehicle. Proposals to close that loophole have died repeatedly since 2010. People who lose their license due to a DUI conviction often use a moped to get around. In a state where public transportation is lacking, legislators didn't want to prevent people from getting to work.
To solve that sticking point, the law creates a special moped license.
Once the law takes effect in November 2018, people who lose their regular license, for whatever reason, can get the separate license, starting the point system over. But the moped license can be suspended too.
"We don't have a lot of sympathy for someone then. If you're driving drunk on a moped, you need to be walking. You're not getting the message," Hembree said Friday.
The law requires people to register their mopeds and attach a license plate, which supporters said creates a way to cut down on theft and identify mopeds in crashes.
Mopeds will still be exempt from property taxes and insurance.
But adding mopeds to the definition of a motor vehicle also closes an insurance loophole. State law requires car insurance to include coverage for uninsured motorists. But if a moped crashed into a vehicle and damaged it, insurers could deny liability, Hembree said.
"This takes care of that," he said.
The law also limits mopeds to roadways where the posted speed limit is less than 55 mph.
In her veto letter last year, Haley said the proposed restrictions for mopeds exceed those for motorcycle drivers.
But state law does require anyone under 21 who's driving or riding on a motorcycle to wear a helmet. The motorcycle advocacy group ABATE - which stands for A Brotherhood Against Totalitarian Enactments - has long squashed any effort to require helmets for older riders.
But ABATE supported adding the helmet provision for mopeds.
Copyright 2018 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.