Helmut Niedermayr was born in Munich. In 1949 he resided in Berlin and raced in both East and West Germany back in the days when the country was divided, but before the Berlin Wall was built (1961).
He made his debut at the end of 1949 at the Sachsenring in East Germany driving a Reif BMW in the 2-litre sports car category. Reif was the official BMW dealer in Chemnitz before the war and he continued as a specialist repair shop after the war. He built an open-wheel "intertype" car for Count Karl von Horn, who entered it for Niedermayr to drive. The "intertype" style permitted it to be entered in sports car races with motorcycle-like wings and lights attached, as well as in Formula 2 events.
In 1950, Niedermayr acquired the car from von Horn and after a minor face-lift, he continued racing the Reif scoring a series of second places in the sports car class at the Schauinsland hill climb, the Solitude race and the Sachsenring.
He started 1951 by competing in the Monte Carlo Rally driving an IFA DKW 900 with Adolf Brudes. On the circuits he appeared on the entry lists driving a BMW-Meteor. However it is not clear whether this was a development of the Reif or a completely new car. It is possible that with cold war raging it was politically expedient not to be seen racing an East German machine. He scored a couple of fourth places in sports car races at Hockenheim and the Nürburgring.
In 1952 Niedermayr switched to a new Veritas RS for the sports car races, perhaps the first such car that Loof had produced in his new premises at the Nürburgring. In Formula 2 his entries were made under the banner of the Renngemeinschaft Halensee, which fielded a Veritas Meteor as well as an AFM 50 for him and on one occasion also for Franz Bumke. It turned out to be a year of contrasts for Niedemayr. He drove an AFM-BMW in the 1952 German Grand Prix at Nurburgring and also drove a Mercedes-Benz 300SL, now with the deeper gull wing doors required by the regulations, at Le Mans with Theo Helfrich finishing second behind the similar car of Lang and Reiss.
In May he entered the ADAC Eifelrennen in a Veritas RS but failed to finish. Then on the 31st August 1952 he competed in his AFM-BMW at the Grenzlandring, a road course around the town of Wegberg. At the end of the first lap he left the road at the Roermonder Kurve. He crashed into the crowd standing on the inside of the track, killing 14 and injuring over 40 people. The local authorities were quick to act, and racing at the circuit was banned.
Helmut eventually returned to the track, appearing briefly in Hans Klenk's Meteor special in the 1954 AVUS GP, and thereafter was seen in a Porsche in rallies and sports car events.
Niedermayr eventually defected to East Germany. He died in 1985 in Cristiansted, Virgin Islands, USA.