"It was good. I was a little disappointed with the back nine, missed a few chances coming in on the last four holes or so but the change in conditions made it quite tough", said McEvoy, who has 11 top tens in 284 European Tour appearances.
"I've been playing very solidly, I'm enjoying being at the top of the leaderboard which is great and I'm not putting too much pressure on myself".
The 24-year-old and McEvoy started the final round as co-leaders, but the American finished with an astonishing six-over 78 after one of the all-time final round chokes in Hamburg.
England's Richard McEvoy is finally a European Tour victor.
DeChambeau was just a shot behind, despite bogeys at two par fives, the 15th and 16th. He won on his 285th European Tour appearance, 17 years after making his debut on the tour and just seven days after triumphing on the developmental European Challenge Tour.
U.S. golfer Bryson DeChambeau reacts after a bad tee shot at the European Porsche Open.
Before Sunday he had won three titles on the Challenge Tour but victory at the highest level had eluded him.
Rather than congratulate McEvoy with any sort of enthusiasm, Dechambeau brushed his rival with a weak handshake and walked angrily past him off the course.
In a tense finish, Blomstrand and Paratore both birdied the last for rounds of 68 and 70 respectively to join John on 278 for the championship and when McEvoy bogeyed the 17th, there was a four-way tie for the lead.