Lang initially was assigned as one of Luigi Fagioli’s racing mechanics. While bedding in the brakes of the Italian’s racecar Lang had a chance to drive the car at close to racing speeds. He liked how it felt. Later back at the factory he was called into Neubauer’s office and was told that Krauss had recommended that Lang be given a trial as a racecar driver. It was through this simple act of kindness by Krauss that a new career was launched. He did well at the trials and later became a reserve driver in addition to his mechanics duties. In 1935 he competed in several races and was able to learn gradually. Both Caracciola and von Brauchitsch looked askance at the mechanic trying to be a racecar driver and unfortunately this was to color their future relationship with their future teammate. Caracciola especially would feel challenged by this upstart, though his reputation as one of the greatest drivers of all time was secure it is never easy for an older athlete to pass the torch to a younger generation. Arguments between the two drivers would continue the rest of their time together and only the intervention of Neubauer would allow for any teamwork. Lang would later say of Caracciola: "Whatever may have come between Caratsch and me, there’s one thing that I must admit quite frankly – he was always my ideal, the greatest driver of them all. There was a tremendous elegance in his driving that no one else equaled. I never stopped studying and admiring him."

Caracciola and von Brauchitsch were drivers from the old school who did not involve themselves in the mechanical workings of their cars. Lang being a mechanic was very much in tune with his cars and thought nothing of grabbing a wrench in later years when he became successful. For this the mechanics adored him and would think nothing of presenting "their" driver with something extra when they prepared his car. 1936 was a lost year for Mercedes but Lang was still able to demonstrate his potential, which made him, sought after by both German teams. He new Prof. Porsche and was asked by him to join Auto Union. Lang stated that since it was Mercedes that gave him his first chance he felt a loyalty to the team, which was rewarded with a new contract.

The team for 1937 would be made up of Caracciola and von Brauchitsch as the senior drivers and Lang joined by Englishman Dick Seaman as the junior drivers. The team would stay divided by this division and it was Neubauer that kept the team from tearing itself apart. The season started at Tripoli, which Lang won by 10 seconds over Rosemeyer for his first victory. At Avus the team prepared some streamlined cars with full bodywork. During practice they tried the cars with wheel covers. Lang was driving one of the cars at close to 400 km/h when the front of the car began to lift and all that he could see was the sky! not a good thing. Slowly, he reduced speed while keeping the front wheels straight. For what seemed like ages he was driving the car on only the two rear wheels before the fronts finally touched down as if landing a plane. Returning to the pits white-faced he convinced the team to remove the wheel covers for the race. When the race was run it was the reluctant aviator Lang that crossed the finish line first. At Brno in Czechoslovakia Lang had his only bad crash of his career when he slid on gravel thrown onto the course by other drivers and flipped his car. Lang was thrown clear and was lucky to escape with only cuts; two spectators were less lucky and died when the car continued out of control to where they had been standing. Any accident involving spectators would bring legal difficulties that were only resolved after the war.

The 1938 season began at Pau with the famous win by Dreyfus in his Delahayes. This was the first season for 3-liter cars and Auto Union was not ready in time for the race. Tripoli was next and still no Auto Union; the death of their star Rosemeyer deeply effected the team. Land won his second race at Tripoli in succession. Reims was up next and Auto Union was there but without Rosemeyer or Stuck and both cars crashed on the first lap! At the Coppa Acerbo Lang suffered an incident that almost had tragic consequences. A rod broke on his car, which in turn severed his fuel line. The spilled fuel caught fire and Lang was just able to jump free of the moving car as his overalls caught fire. After rolling in the grass his close or what was left of them had been extinguished. Just then Rene Dreyfus, who was suffering from transmission problems, drove by and the two of them returned to the pits. The Mercedes team upon seeing their missing driver leapt with joy.

Lang was fully competitive with the other drivers except when it came to rain. Caracciola was the accepted regenmeister but Lang was still not comfortable in those conditions. It took extra practice under the tutelage of Neubauer in order for Lang to gain more confidence. 1939 Belonged to Lang and as the final race approached he was in the lead for the European Championship. The last race was the Swiss GP, which he won after a duel with Farina in an Alfa and Caracciola. Lang also won the German Hillclimb Championship that same year. Lang was at the peak of his career winning five of the eight races entered but the war interrupted his career and by the time it was over he had lost what should have been his best years. His last major victory came at the Le Mans 24 Hours race in 1952 while driving a new Mercedes 300 SL.