Ahlmann and his experienced horse, Codex One, won a three-horse jump-off over eight fences in 40.56 seconds at Berlin's Sommergarten.
"It's an amazing day for us, Codex is back after a long time, directly in the first Grand Prix with the win," Ahlmann said in a televised interview afterwards. "He jumped amazingly."
The horse, a 15-year-old Hanoverian stallion owned by Ahlmann's long-time supporter Marion Jauss, had missed last year's Rio de Janeiro Olympics due to injury.
"For the show, for me and for Marion Jauss, she is from Berlin, the whole package is unbelievably good," Ahlmann added.
The 11th leg of the LGCT had attracted eight of the top ten riders, who competed in a new venue in the heart of the German capital.
The capacity crowd watching from the grassy banks of the Sommergarten roared when Ahlmann jumped the final fence just 0.45 seconds ahead of fellow German Simone Blum and DSP Alice. Head-to-head pictures revealed afterwards that the two-time Olymic bronze medalist had been leading Blum from the start.
Holland's Leopold van Asten and VDL Groep Beauty finished third with eight penalties.
Ahlmann's win handed him 40 points, meaning he moves up one spot to third place on the overall leaderboard of the Longines Global Champions Tour with 220 points.
"I'm really focused on the Global Tour and I try my best and this was a quite important and big step," said Ahlmann.
Smolders remains in the lead
Although Holland's Harrie Smolders remains the overall leader with 252 points, a disappointing 11th finish in Berlin saw his lead over Italy's Alberto Zorzi shrink to just 31 points.
Smolders had come to Berlin in top form, having clinched a double win two weeks ago in Chantilly, where he took both the LGCT Grand Prix and the team competition with Hamburg Diamonds.
But the transition from the vast grass arena of Chantilly to the compact sand amphitheater in Berlin's Sommergarten proved a bridge too far for Smolders, who collected five penalties with his 12-year-old Holsteiner stallion Capital Cornado.
Although German Olympic showjumping legend Ludger Beerbaum delivered a clear round with his new horse Chacon, he collected a point penalty for finishing just 0.28 seconds outside the allowed time of 68 seconds on a technical course set by Frank Rothenberger. Beerbaum, a four-time Olympic champion, eventually finished in fourth place.
Global Champions League
Earlier in the day, Italy's Zorzi and Ireland's Bertram Allen handed reigning Global Champions League winners Valkenswaard United a win in Berlin to close in on overall leaders Hamburg Diamonds.
Zorzi and his grey Hanoverian gelding Cornetto K produced two clear rounds while Allen and Izzy By Piccobello knocked down one fence.
With only four more events to go, the team competition is hotting up with Hamburg Diamonds just 8 points clear of Valkenswaard United, and 33 points ahead of Mexico Amigos in third place.
The next leg of the LGCT and GCL will take place next weekend at the Royal Hospital Chelsea in the heart of London.