The Germany and Poland internationals struck to give BVB a winning experience to take into the game of their lives against Real Madrid in the Champions League
The upcoming visit of Jose Mourinho's men in the Champions League loomed large and threatened to overshadow events at Signal Iduna Park, but Marco Reus made sure that BVB's heads were kept in the right place with an opportunistic opener in the first minute of play.
Dortmund would spurn further chances, especially in the first half, to extend their lead but Robert Lewandowski's goal three minutes from time eventually ended the meek challenge of a Mainz outfit that were simply outclassed for long spells.
Jurgen Klopp surprised many by fielding a full-strength team, with Nuri Sahin the only player in the XI not considered an automatic starter, after beating Sven Bender and Sebastian Kehl to a midfield berth.
And Dortmund could not have wished for a better start when, with 30 seconds on the clock, Christian Wetklo spilled a routine catch, gifting Reus the opportunity to turn home the loose ball.
The Mainz keeper would begin to make amends for his mistake, however, when he reacted well to thwart another effort from Reus from the edge of the box, before saving from Neven Subotic from close.
It was one-way traffic at Signal Iduna Park, with Mainz barely able to get a touch on the ball as Dortmund kept them penned back deep inside their own half.
Nikita Rukavytsya snatched at a rare half-chance for the visitors, but all the real threat came from the home side, as Lewandowski bent a shot inches off the top corner after good link up play from Mario Gotze and Ilkay Gundogan.
Dortmund’s pressure was incessant but, as the half wore on, their cutting edge dried up ever so slightly, allowing Mainz a little time to regain some semblance of shape, though early in the second half Marcel Schmelzer fired a pair of back-to-back efforts agonisingly wide to remind the visitors just who were in charge.
However, with the game maturing past the hour mark, the home side seemed less and less interested in pressing their advantage, as the tempo on the pitch dropped markedly with long spells of leisurely Dortmund possession.
And Adam Szalai almost made them pay for their lack of urgency with a quarter of an hour to go, when his finishing touch on a crisp passing move grazed the outside of Roman Weidenfeller's post.
But the visitors could not push on from that encouraging nearly-moment, and instead it was Dortmund who would find a second, with Lewandowski wrapping up the points from a few yards out after neat combination play from Lukasz Piszczek and Moritz Leitner.