The Bavarians are the only side left in the Bundesliga yet to drop points this season.
The first goal arrived as early as the seventh minute, with Joshua Kimmich heading home from James Rodriguez's corner.
Lewandowski got his name on the score-sheet in the 24th minute from the penalty spot, after Rodriguez was brought down in the box.
The second half produced no goals at the VELTINS-Arena, as Niko Kovac's side ran out comfortable winners on the day.
The Bayern coach was full of praise for his side's display on the road, which saw them reclaim top spot.
"We saw a very, very good Bundesliga match today," Kovac told Bayern's official website.
"My team were dominant over 90 minutes, they were in control and created many chances. Unfortunately they scored too few goals.
"[It was] a great performance. I have to praise the whole team today, what we did up front but also in defence, without the ball. We regained possession quickly when we lost it up front."