Still nursing his way back after missing several months last year due to an ankle injury, the German opened his campaign with a hard-fought five-set win over Juan Pablo Varillas on Tuesday.
The 12th seed failed to up his game in the second round on Margaret Court Arena as he went down 6-7 (1-7), 6-4, 6-3, 6-2 against the American.
To add insult to injury, a bird decided his head was a good place to poo on midway through the match.
After making a solid start against Mmoh by winning the opening set, things went downhill for the former world No 2 as the American broke twice in the second set, twice in the third set and three times in the fourth set.
Zverev made 58 unforced errors compared to 57 winners while Mmoh hit 42 winners with 35 unforced errors.
"Wow, this is the biggest win of my career without a doubt," said Mmoh. "Life is crazy, this is insane."
Up next is his compatriot JJ Wolf, who upset 23rd-seeded Argentine Diego Schwartzman in three sets.
Although it was a good day at the office for Americans with Jenson Brooksby beating second seed Casper Ruud and Ben Shelton defeating Nicolas Jarry, there was big defeat for Taylor Fritz as the eight seed was beaten 6-7 (4-7), 7-6 (7-2), 6-4, 6-7 (6-8), 6-2 by Australian wildcard Alexei Popyrin.
"Oh my God, it was just crazy man. I really couldn't have done it without you guys," the local hero told those at John Cain Arena after his win.
"This win means so much to me. I had the toughest year last year, didn't win many matches. I won as many matches this year as the whole of last year and it's only January.
"I don't want that feeling last year that I had ever again. I wrote it down. I'm going to keep going and try and go all the way because I love this feeling and I want more of this feeling.
"I want you guys to have this feeling more. Man I love you guys so much. This is a dream for me and I don't want to wake up at all."
Brooksby and Popyrin will meet next and the winner will take on either Wolff or Mmoh in the fourth round.