The departure of Thomas, who played in a total of 13 games the last two seasons while spending much time in coach Joe Paterno's doghouse, leaves the Nittany Lions with just three scholarship upperclassmen, Stephon Morris, Adrian Amos, and Mike Wallace, at cornerback.
Issah, rated four stars by Scout.com and three stars by Rivals.com, is the 14th member of the incoming 2013 freshman class to commit to Penn State and the sixth this month. He received a scholarship offer from Penn State in February but a visit to campus with his parents two weeks ago helped him make up his mind.
"Penn State is the right fit," Issah told ESPN.com.
Issah recorded more than 100 tackles and 14 sacks last season, and rushed for more than 1,000 yards and 31 touchdowns for Central Dauphin.
Thomas, of Greenbelt, Md., had been listed as even with Amos for the No. 1 job at right cornerback after the end of spring practice. Instead, he became the fourth player to part ways with the team since O'Brien took over in January, and joins former West Catholic star Curtis Drake as the second cornerback to leave.
Thomas had academic issues the last two years and also had been cited by State College police for disorderly conduct in an off-campus scuffle in February 2011.
The Lions also have two incoming freshmen and three walk-ons at the cornerback position.
Contact Joe Juliano at 215-854-4494 or firstname.lastname@example.org or follow him on Twitter @joejulesinq.