St. Clair County Circuit Judge Stephen McGlynn was sworn into office by Chief Judge John Baricevic on Dec. 1.
He will serve a six year term.
McGlynn, Republican, narrowly defeated Associate Judge Heinz Rudolf, Democrat, in the Nov. 4 election by a margin of 50.53 to 49.47 percent.
McGlynn received 37,875 votes to Rudolf's 37,086 votes, a difference of 789.
The two were vying for the circuit court seat that had been vacated by former judge Michael Cook, who is serving a two-year federal sentence on heroin possession and weapons charges.