Kopec captured the seat with a 4-1 vote with little discussion. Republican Supervisor Ally Miller voted against him.
Kopec, who has worked on many local campaigns and as an aide to Tucson City Councilwoman Karin Uhlich, beat out fellow Democrats Pamela Powers Hannley and Ted Prezelski.
Hannley is already running for a seat in LD9, which includes the north side of Tucson, the Catalina Foothills and Casas Adobes.
The second seat in LD9 is held by Democrat Randy Friese, an emergency-room doctor.