The Reds will fly to Germany just one day after a glamour friendly with Barcelona at Wembley Stadium, allowing manager Jürgen Klopp to return to the club where he spent 18 years as a player and coach.
The German remains a legend at his ex-club, having guided them to the Bundesliga, where he then took them into Europe for the first time ever under the Europa League's previous alias, the UEFA Cup.
Mainz, now under the management of Christian Heidel, finished sixth in Germany's first division last season - sealing their automatic qualification to the Europa League group stages for next term.
Liverpool's trip to the Opel Arena is their ninth warm-up fixture prior to the 2016-17 Premier League season, with the club kicking off their campaign away at Arsenal just six days afterwards.
Mainz recently sold goalkeeper Loris Karius to Liverpool after the Merseyside outfit activated his £4.7 million release clause, although the 22-year-old is expected to join up with Germany at the 2016 Olympics in Rio, Brazil, rather than return to his former club.
Liverpool's lengthy pre-season starts with several local domestic fixtures against Football League opposition, before the squad jet off to the United States to take part in the International Champions Cup. Facing Chelsea and AC Milan, they will also take on Roma for a third time in four years in an unrelated friendly.
Just four days after their final fixture in America, Klopp and co. return to England to take on Barcelona at a sold-out Wembley in their penultimate friendly - with less than 24 hours between that and their trip to the Rhineland to play against Mainz.
The club's chief commerical officer Billy Hogan insisted that Liverpool's fanbase has "grown significantly" in Germany and therefore a trip there represents "a fantasic opportunity" for the fans "to see the Reds play a Bundesliga team."
Liverpool last faced Mainz 10 years ago at their old stadium, the 20,300-seater Stadion am Bruchweg. Rafael Benitez's men losing 5-0 to a Mainz side managed by none other than Klopp himself as they prepared for a third-round Champions League qualifier with Maccabi Haifa just days later.
It marks the end of a gruelling pre-season for the Reds' squad, with Klopp reportedly introducing triple training sessions each day to focus on the fitness of his players and ensure they are ready for his demanding style-of-play throughout the year.
Though the fixtures are still some weeks away, Klopp is likely to seperate his squad into two different teams for their final friendlies - with half returning to Liverpool after playing Barcelona and the other travelling to the manager's homeland.
Liverpool's full 2016-17 pre-season fixture list:
Friday July 8 - Tranmere Rovers, Prenton Park, 7pm KO.
Wednesday July 13 - Fleetwood Town, Highbury Stadium, 7pm KO.
Sunday July 17 - Wigan Athletic, DW Stadium, 4pm KO.
Wednesday July 20 - Huddersfield Town, John Smith's Stadium, 7:45pm KO.
Thursday July 28 - Chelsea, Rose Bowl, 4:30am KO.
Sunday July 31 - AC Milan, Levi's Stadium, 3am KO.
Tuesday August 2 - AS Roma, Busch Stadium, 1:30am KO.
Saturday August 6 - Barcelona, Wembley Stadium, 5pm KO.
Sunday August 7 - Mainz, Opel Arena, 2:45pm KO.