The actor will now appear as a series regular in the CBS legal drama, which stars Julianna Margulies.
He told Entertainment Weekly: "[Eli is] more heavily involved in Peter's campaign [this season]. He gets meaner and [more] ruthless."
He also explained that he was relieved to be promoted to a regular cast member.
"I sort of have an idea of where my character is going now," he claimed. "[Before] I had no idea what was going to happen. The next week I could've been a murderer."
Cumming recently received an Emmy nomination for the role of Eli, but lost out to Dexter's John Lithgow.
The Good Wife returns to CBS on September 28.