The 7-2 decision allows voters to continue casting mail-in ballots under existing rules. The ruling comes as Trump continues to spread unfounded allegations of fraud in mail-in ballots.
For some voters, the back-and-forth has created confusion.
The issue was highlighted Tuesday, National Voter Registration Day, as local officials encouraged residents to register to vote and learn more about available voting methods.
At a voting event aimed at increasing participation among young people, Philadelphia City Commissioners Chairman Omar Sabir pointed to the concern over registration rates among new voters.
"There's a big vacuum of 18-year-olds who are not registered to vote. We want to see you register," Sabir said.
Some advocates say public discussion surrounding mail-in ballots has left voters uncertain about the process.
"It has led to a lot of confusion," said Lauren Cristella, president of the Committee of Seventy.
Cristella said the Supreme Court's ruling means voters can continue using the same mail-in voting process already in place.
"The Supreme Court decided that the process you know is the process you'll use," Cristella said. "Nothing about how we vote by mail in Pennsylvania, or anywhere else in the country, is changing."
Cristella, whose organization is a nonpartisan, nonprofit group, said voting by mail remains secure. However, opinions among voters remain divided.
"I don't think you should be able to send a mail-in ballot if you can't make it to a voting office," said West Philadelphia resident Kier James.
Another voter who didn't want to share her name expressed support for the option.
"I think the mail-in ballots are great for those who can't make it to the polls, for those who have to work," she said.
Pennsylvania voters interested in voting by mail can request a mail-in ballot through Oct. 27. Ballots can be requested online or in person at an elections office or satellite elections office.
"They'll hand it to you right on the spot. You can even vote there and hand it right back. Or you can take it home, vote at your kitchen table. Drop it off at a drop box or in the mail," Cristella said.
Election offices also serve as locations where residents can register to vote.
"We want you to use our satellite election offices. We want you to walk in, get registered, then request your ballot is done!" Sabir said.
Residents may also register online, through the Department of Motor Vehicles, or at voter registration drives, including an event held at Community College of Philadelphia.
Pennsylvanians have until midnight on Oct. 19 to register to vote.
Election officials and voting advocates urged voters not to wait until the last minute to request or return a ballot.
For voters who are already registered, the Committee of Seventy recommends checking voter registration information twice a year to confirm that names, addresses and polling locations are accurate before Election Day.