Andre Breitenreiter guided Paderborn to promotion from the 2. Bundesliga in his debut campaign last season and the north-west club were on course to mark their maiden top-flight fixture with a win on Sunday.
Shinji Okazaki gave visitors Mainz a 33rd-minute lead by tapping home from close range after Paderborn keeper Lukas Kruse failed to hold on to a free-kick.
Paderborn's response was swift, though, and Elias Kachunga netted the club's first Bundesliga goal four minutes later with a fine finish from 20 yards.
The Japan international duly picked himself up to convert from 12 yards, giving new Mainz coach Kasper Hjulmand a point in his Bundesliga bow.
Sunday's other top-flight game also saw late drama as Borussia Monchengladbach salvaged a 1-1 draw in the 90th minute at home to Stuttgart.
Branimir Hrgota, who scored twice against Sarajevo in the UEFA Europa League in midweek, should have put the hosts ahead just after the break.
Armin Veh's men could not hold on for the win, though, as Monchengladbach's World Cup winner Christoph Kramer fired in a late equaliser after getting on the end of Alvaro Dominguez's well-placed cut-back.
