Ronnie O'Sullivan of England reacts in the first round match against James Cahill of England during day four of the 2019 Betfred World Snooker Championship at Crucible Theatre on April 23, 2019 in Sheffield, England
The Rocket led 2-1 and 3-2 but some uncharacteristic loose play from O’Sullivan allowed the world number 70 Mei back in to snatch the last two frames to seal the win.
O’Sullivan, whose English Open challenge has been marked by inconsistency, looked on course for the quarter-finals when a majestically crafted 134 put him within a frame of victory.
O'Sullivan makes a clearance of 134
However, the 43-year-old could only muster a total of 30 points in the final two frames as Mei showed excellent composure to take the biggest scalp of his career.
Mei's shock win means the world's top four players - Judd Trump, O'Sullivan, Mark Williams and Neil Robertson - have all been dumped out of the first Home Nations tournament of the year in the last 24 hours.
Watch: The moment Mei sank O'Sullivan at the English Open
Across the room, Mark Allen racked up two centuries and two half-centuries to beat Barry Hawkins 4-0 and reach the quarter-finals.
Both players struggled to adapt to the conditions early on with the eventual victor admitting the white "played heavy" in the early stages.
But it was Allen, now one of the heavy favourites for the English Open title, found his form quickly, making a break of 60 to take the opening frame.
Hawkins then gifted him an opportunity in the second as he missed the red as he split the pack and Allen navigated the table for 89 and a 2-0 lead.
The Pistol then hit his straps with clearances of 100 and 129 to face the winner of Lee Walker vs Gary Wilson.