Janowicz was out of sorts in Halle against the world number 135 - double-faulting 13 times en route to a 7-6 (7-5) 6-2 defeat at the ATP 250 grass-court tournament.
Herbert began positively and was soon 5-1 up thanks to two breaks of serve. Even when Janowicz, the only seed in action on Monday, recovered to take the opening set to a tie-break, the underdog kept cool to move ahead.
Things soon got worse for Janowicz as Herbert breezed through the second set, breaking on two further occasions to set up a second-round clash with Philipp Kohlschreiber.
Home favourite and winner of the competition in 2011, Kohlschreiber enjoyed a comfortable day as he swept aside Italian Andreas Seppi in an hour and nine minutes.
Kohlschreiber was quickly three games to the good and Seppi failed to claw his way back into the first set.
The world number 27 then claimed an early break in the second before holding serve thereafter to complete a 6-3 6-4 victory.
Kohlschreiber's compatriot Dustin Brown is another competitor safely through to the second round, with the pair on course for a quarter-final meeting.
Wildcard Brown came through 6-3 6-4 against Russian qualifier Andrey Kuznetsov, having come back from two games down in the second set.
Brown will meet recently crowned French Open champion and top seed Rafael Nadal in the next stage, while the Halle crowd saw another German progress in the form of wildcard Peter Gojowczyk.
The world number 120 was too strong for Poland's Michal Przysiezny, with a break in each set enough to secure a 7-5 6-4 win.
Jan-Lennard Struff was the only German to crash out on Monday as he lost to Joao Sousa in straight sets.
Struff, a wildcard entrant, was unable to cope with the Portuguese world number 47 as he went down 6-4 6-2 in an hour and 12 minutes.