The world number one, riding Codex One, edged out compatriot Daniel Deusser and Frenchman Roger-Yves Bost in a tight jump-off to claim a hard-fought victory.
Another German, Janne-Friederike Meyer with Cellagon Lambrasco, finished fourth after opening the finale with a clear round in a time of 52.65 seconds.
Deusser with Cornet d'Amour went next and set a new benchmark time of 50.25s before Bost, atop Nippon d'Elle – whose breeder Alexis Pignolet passed away on Friday – slotted into provisional second having stopped the clock at 52.25s.
But Ahlmann eclipsed them all with a majestic round of 49.33s, claiming first place and, once Ukrainian Katharina Offel had knocked down two fences in her final effort, victory.
“It felt fantastic. I had a really great day today,” Ahlmann said. “Codex One did an amazing job in the three rounds. He really showed everybody how special he is.
“In the jump off I took a lot of risks. I was in a position to see Daniel (Deusser) and it was clear I had to take the last risk. Today was my lucky day. I will try to compete the whole season (of the Longines Global Champions Tour) as much as possible.”
Ahlmann’s win takes him onto 40 points from three events so far this campaign, but it is Pius Schwizer who tops the rankings after his strong opening performance in Madrid and a 10th-placed finish in Hamburg.
Next stop on the Tour is at Wiesbaden, also in Germany, next weekend.