Elversberg had opened the scoring in two of their three home games so far this term and did so again after just seven minutes.
After the home side were awarded a free-kick, Felix Keidel stepped up to curl home his second goal of the season from 20 yards, having previously scored a free-kick in their rollercoaster 4-3 win at Borussia Monchengladbach in September.
That was just the fourth away goal that Mainz had conceded so far this season, but it was soon five shortly before the half-hour mark.
After a period of Mainz possession, Elversberg broke forward as David Mokwa Ntusu found Francis Onyeka, who showed great strength to create space, power into the box, and fire a low left-footed strike past Robin Zentner to score his first Bundesliga goal.
The hosts’ intent didn’t finish there as Keidel, Mokwa Ntusu, and Lukas Petkov all fired over from range before Maurice Krattenmacher capped off a sensational 45 minutes for Vincent Wagner’s side in first-half stoppage time.
A Keidel corner caused chaos inside the Mainz box before Elversberg’s top goalscorer Krattenmacher ghosted away from two defenders, cutting onto his left foot and sweeping a composed first into the far corner.
Mainz were a shadow of the side that boasted the Bundesliga’s only 100% away record coming into the game and looked to respond after the break as Phillip Tietz’s header was saved by Nicolas Kristof.
The away side then thought they’d begun their fightback when substitute Ransford Konigsdorffer looped a clinical finish into the corner two minutes after coming on, only for a VAR review to find the striker had run onto the ball from an offside position.
Elversberg then nearly made the scoreline even more emphatic as only an excellent save from Zentner prevented Mokwa Ntusu from adding a fourth after a strong solo run.
A fourth goal did finally arrive in stoppage time when Cole Campbell collected a cross on his chest before acrobatically volleying the ball into the bottom corner.
The only thing missing from Elversberg’s display was a first clean sheet at home in the Bundesliga, a record which continued as Mainz managed a consolation less than a minute later as Benedict Hollerbach tapped in from close range after Konigsdorffer’s initial effort hit the post.
Despite the consolation, Mainz’s evening ended with further misery moments later, however, as Stefan Posch received his second yellow card with almost the very last action of the match.
Elversberg’s biggest ever Bundesliga win takes the top-flight newcomers to the seemingly inconceivable heights of the UEFA Europa League position in fifth as their incredible rise continues.
Mainz, however, have now lost four of their last five matches in all competitions after a positive start to the campaign, with the daunting prospect of hosting the record champions Bayern Munich to come next weekend.
Flashscore Man of the Match: Francis Onyeka (Elversberg)
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