Mendenhall ran for 146 yards and a touchdown, Roethlisberger threw for 200 yards and a score, and the Steelers defense held the Jaguars to just 209 total yards for a 17-13 win.
Gabbert had a miserable day, completing just 12 of his 26 throws for 109 yards and a touchdown. Maurice Jones-Drew ran for 96 yards for the Jags.
Pittsburgh scored the first 17 points of the game in the first half, then held on as its offense went scoreless in the second half.
With the win, the Steelers move to 4-2 on the season, while the Jaguars fall to 1-5 with the loss.
For more on the Steelers, head to SB Nation's Behind the Steel Curtain; for more on the Jaguars, head to Big Cat Country. For Week 6 NFL scores and news, Week 6 NFL video highlights, and Week 6 fantasy football stats and updates visit our comprehensive StoryStreams.