Both teams will hope to avoid a relegation battle this time around, with Mainz finishing 13th and just two points above the drop zone, while Union finished 17th and only stayed up on goal difference.
Match preview
A clear problem last season for Mainz was their goal return having scored just 39 times in 34 games; the fourth-lowest tally in the Bundesliga.
In terms of transfers, Mainz have sold a number of players in the summer and winger Brajan Gruda recently joined Brighton & Hove Albion too, so attacking reinforcements will be needed if they are to avoid another repeat of last season's low goal tally.
Manager Bo Henriksen did however improve Mainz's fortunes after joining in February and his side go into Saturday's match on the back of a nine-game unbeaten league run, including a 3-0 home win over Borussia Dortmund on the final day of the season.
In their last game, Mainz needed extra time to win 3-1 away at neighbours Wehen Wiesbaden in the DFB-Pokal first round, where goals from Dominik Kohr, Jonathan Burkardt and Nadiem Amiri saw them narrowly progress.
Now with a full pre-season under Henriksen, the question is whether we will see the continuation of their impressive form from the end of last season, and a home game against a side that finished beneath them last season gives Mainz a good chance of opening their league campaign with a positive result.
Union stayed in the Bundesliga by the skin of their teeth last season and they were the third-lowest scorers in the division having only scored 33 goals, six fewer than Saturday's opponents.
It took a dramatic injury-time winner against Freiburg to keep their Bundesliga status and it is hard to believe that in the previous season they finished fourth in the table, but some of this could be attributed to their participation in last season's Champions League.
The away side do however begin this campaign with their new head coach Bo Svensson, and his first league game just happens to be against his former team; the Dane having managed Mainz from 2021 to 2023.
Svensson's side come into Saturday's game on the back of a 1-0 win over non-league outfit Greifswald, with a solitary Yorbe Vertessen goal settling the tie.
It was a case of profligacy as Die Eisernen were wasteful in front of goal, which is a far cry from their pre-season results which saw them score 12 times in five games.
Mainz 05 form:
Union Berlin form:
Team News
We do not expect to see many changes from the side that played against Wehen Wiesbaden, and with no injury concerns the manager has a fully-fit squad to pick from here.
In terms of options available if Henriksen does look to make a few changes, the likes of Paul Nebel and Marco Richter could push for a start in the midfield.
Union do not have the same luxury as their opponents, with defenders Josip Juranovic (hamstring) and Jerome Roussillon (muscle) unavailable here.
Kevin Volland (knee) is also set to miss out, but in terms of attacking options Svensson can still choose from Jordan Pefok, Ivan Prtajin and Benedict Hollerbach to lead the line on Saturday.
The away side will also be hoping to have Robin Gosens available amid ongoing rumours linking the wing-back with a move to Crystal Palace.
Mainz 05 possible starting lineup:
Zentner; Kohr, Leitsch, Hanche-Olsen; Caci, Sano, Amiri, Mwene; Sieb, Lee, Burkardt
Union Berlin possible starting lineup:
Ronnow; Doekhi, Vogt, Querfeld; Haberer, Schafer, Khedira, Gosens; Benes, Vertessen; Pefok
We say: Mainz 05 1-0 Union Berlin
Mainz come into this season with great momentum and we expect that to bear fruit on Saturday at home as they look to continue their fine league form.