Jancek will serve alongside Willie Martinez, who has held the title of defensive coordinator since 2005, but came under fire a year ago when the Bulldogs' defense struggled down the stretch, giving up 38 or more points in four of its final five regular-season games.
"Willie Martinez is our defensive coordinator, and as co-coordinator, John has been and will continue to be instrumental in our game planning on the defensive side of the ball," Georgia head coach Mark Richt said in a statement released by the school Tuesday.
Jancek is the third Georgia assistant to turn down jobs with other schools this offseason.
Defensive line coach and recruiting coordinator Rodney Garner was wooed by Tennessee after Lane Kiffin took over the Volunteers' program in January and was offered a significant raise. Garner chose to remain with Georgia, however, citing loyalty to Richt and the Georgia program.
Offensive line coach Stacy Searels interviewed for a position with Auburn, his alma mater, prior to Georgia's bowl game against Michigan State, but he, too, decided to remain in Athens. Searels was later given the title of running game coordinator in addition to his duties overseeing the offensive line.
Jancek's promotion comes with little explanation from Richt as to who will be in charge of the day-to-day or game-day decision-making on defense, but Jancek told the Athens Banner-Herald that he did not anticipate significant changes from what the Bulldogs had done in the past.
Richt will speak with media today as part of a previously scheduled news conference to discuss the start of spring practice, which begins March 17.
Jancek joined the Georgia staff in 2005 after serving as defensive coordinator at Central Michigan. He is a graduate of Grand Valley State.