In 1931 he joined Mercedes-Benz, working under Fritz Nallinger on the development of the Mercedes 170V. In 1936 Uhlenhaut assumed leadership of the race car department. Auto Union dominated the 1936 Grand Prix season over the aging Mercedes-Benz W25. As a talented driver in his own right, he was able to spot the deficiencies of the chassis and suspension, however he never raced competitively as he was needed for his engineering skills. The replacement for the W25, the Mercedes-Benz W125, remedied the chassis and suspension shortcomings and was much more powerful. The W125 dominated the 1937 Grand Prix season, and was considered the most powerful Grand Prix car until the turbocharged cars of the early 1980s. In 1938 rule changes necessitated a new car for 1938 Grand Prix season, the Mercedes-Benz W154.
Based on the Mercedes-Benz 300 SLR sport racer of 1955, Uhlenhaut created a road legal SLR/SL hybrid. Capable of approaching 290 km/h (180 mph), the 300 SLR Uhlenhaut Coupé easily earned the reputation of being the era's fastest road car. A story circulates that running late for a meeting Uhlenhaut roared up the autobahn from Munich to Stuttgart in just over an hour, a 137 mile/220 km journey that today takes two-and-a-half.
Rudolf Uhlenhaut, who never owned a car of his own, retired in 1972. He needed hearing aids, possibly due to damage caused by his loud cars.