Marco Reus and Shinji Kagawa found themselves on the scoresheet on either side of the break to keep the gap between Dortmund and Bayern Munich at five points.
Tough first half, but Reus breaks the deadlock
It took over 20 minutes for either side to grow into the game and develop a real opportunity to break the deadlock. The home side would threaten first as Marcel Schmelzer hit an effort from the edge of the box after 23 minutes played but the shot was saved well by Loris Karius.
The visitors had a chance of their own just before the half hour mark as a set piece was whipped in and met the head of Leon Balogun but the header flew wide of the post. Mainz were made to pay for their missed chance as Dortmund would open the scoring a couple of minutes later on the half hour mark.
A well-weighted ball from Gonzalo Castro found Marco Reus in acres of space inside the penalty area and the German international cooly powered a shot low and hard into the bottom corner to make it 1-0.
A lapse in concentration benefitted the hosts as Mainz had played an impressive defensive game in first 30 minutes.
Kagawa kills any hopes of a comeback
The second half saw Dortmund come out flying with a chance just a minute in the second 45. Shinji Kagawa's shot from the centre of the box was again stopped well to keep the deficit at one.
The home side looked the better of two once again and doubled their lead 17 minutes from full time. Henrikh Mkhitaryan found Pierre-Emerick Aubameyang inside the box and the striker crossed for the onrushing Kagawa, who made no mistake in front of an empty net to make it 2-0.
Mainz could only go in search of a goal to make things interesting but could not find a foothold in the match. Dortmund dominated possession and were the more likely to add a third in the last 15 minutes.
Aubameyang was close to getting in on the act with a minute left in normal time but his shot was saved comfortably by Karius. On another day. Dortmund could've won this meeting a lot more convincingly.
Dortmund would eventually close it out and win 2-0, cruising to what was a comfortable win in the end for Thomas Tuchel's side. A clinical finish from Reus in the first half and a simple tap-in for Kagawa late in the second half hands Dortmund three points, keeping them within five points of Bayern Munich, who won earlier today.
Tuchel's side are comfortably in second while Mainz remain in sixth place and will be disappointed in their performance against of the league's top sides. Dortmund deserved their goals and the win while Mainz failed to register a single shot on target.