Native fighter Nielsen took all three scorecards by 117-110, 117-110 and 117-111 in an assertive performance over the Reading-based Gambian in front of 2,131 fans at Blue Water Dokken to retain his WBA Intercontinental middleweight belt.
Former Prizefighter winner Mendy recovered well from a fifth-round knockdown after catching a strong Nielsen right to the chin, but could not breach the Dane’s controlling jab.
The 21-year-old improved his record to 18-0 with the win, while ‘The Tiger’ picked up his sixth professional defeat to 14 victories.
“I am happy with my performance,” Nielsen said. “This is a good way to start the new year.”
Promoter Nisse Sauerland said: “We knew that Mendy would start out strongly but that Patrick would eventually outbox him. This was a good win for him.”
Elsewhere on the card, Alexander Miskirtchian successfully defended the European featherweight title with a 12th-round stoppage of Andreas Evensen, a former victim of Ricky Burns.
The Colombian-born Norwegian bounced back after being knocked down in the third by the Armenian-Belgian, but the fight was waved off after Miskirtchian felled Evensen again in the final round.
And Londoner Deion Jumah, a two-time ABA senior champion and recent recruit to the Sauerland camp, won his second fight in eight days with a cruiserweight win over Igor Pylypenko to go 2-0 as a pro following his switch from the amateur ranks.
