Eddie Howe's side ran out 3-0 winners over 1860 Munich on Friday night, while the Bundesliga outfit went down 1-0 to Turkish giants Besiktas.
Match preview
A goal from the forgotten man himself in Cenk Tosun proved to be the difference in Mainz and Besiktas' friendly encounter on Friday, as the ex-Everton man struck a first-time effort into the far corner with 24 minutes gone to settle the contest.
Bo Svensson's side had eased to a 4-0 success over Wormatia Worms before being quelled by Besiktas, and the German top-tier side will finalise their pre-season preparations versus Athletic Bilbao on July 23 before they seek to build on last season's eighth-placed Bundesliga finish.
Such a standing marked Mainz's best finish in the Bundesliga since they placed sixth in the 2015-16 season, and Svensson has already overseen a major internal reshuffle in the summer months, with defensive outgoings aplenty at the Mewa Arena.
Moussa Niakhate has headlined a plethora of departures that also includes Daniel Brosinski, Jeremiah St Juste, Kevin Stoger and Luca Kilian, with Angelo Fulgini, Anthony Caci, Danny Da Costa and Aymen Barkok among those to arrive to help steady the ship.
In contrast, Newcastle started as they mean to go in Europe in front of a 600-strong crowd at the Saalfelden Arena, as Eddie Howe oversaw a convincing 3-0 success over 1860 Munich in Austria.
The midfield trio of Joe Willock, Sean Longstaff and Bruno Guimaraes were all on target in the second half for the Magpies, who are now two for two in pre-season after thrashing neighbours Gateshead 5-1 at Darsley Park last Saturday.
Predictably, Newcastle have not hesitated to splash the cash this summer and even managed to beat AC Milan to the signing of Sven Botman - a Ligue 1 title winner with Lille - while Nick Pope and Matt Targett have also signed permanent deals at St James' Park.
The Magpies' summer business should not end there by any means, as with the financial backing of the much-maligned owners, Howe is in the process of building a Newcastle squad that he hopes to be able to steer back into European football after a prolonged absence.
Facing Benfica, Atalanta BC and Athletic Bilbao will be a good test of Newcastle's European mettle before the new Premier League season kicks off, where Howe's side will aim to give Nottingham Forest a harsh welcome back to life in the big time.
Mainz 05 pre-season friendlies form:
Newcastle United pre-season friendlies form:
Team News
Mainz have been handed a major injury boost with the news that Switzerland right-back Silvan Widmer is now back in team training, but it remains to be seen if the 29-year-old will be fit to feature on Monday.
Playmaker Lee Jae-sung is still struggling with a knee injury and is unlikely to feature, while midfielder Niklas Tauer did not travel to the training camp after contracting COVID-19.
Fulgini is part of the crop and could make his friendly debut for the hosts this week, while another new arrival in Marcus Ingvartsen ought to spearhead the attack.
Meanwhile, Newcastle midfield pair Willock and Jonjo Shelvey have overcome their injury woes to be available for pre-season as expected, but Javi Manquillo is reportedly undergoing specialist treatment for an injury back in his native Spain.
Howe is otherwise working with a full complement of options for his side's European tour and made full use of his squad for the 1860 Munich win, with Pope and Botman both playing one half each.
Guimaraes and Longstaff may now force their way into the starting lineup after their goals off the bench last time out, which could see 19-year-old Elliot Anderson cede his spot in the middle.
Mainz 05 possible starting lineup:
Dahmen; Leitsch, Bell, Hack; Lucoqui, Kohr, Fernandes, Martin; Fulgini; Ingvartsen, Mustapha
Newcastle United possible starting lineup:
Pope; Trippier, Botman, Lascelles, Targett; Longstaff, Shelvey, Guimaraes; Fraser, Wilson, Saint-Maximin
We say: Mainz 05 0-2 Newcastle United
Most of Newcastle's signings since their takeover have had desired impact so far, and the additions of Pope and Botman have only beefed up a Magpies defence that a youthful Mainz attack will struggle to break down.