Dry witted and even tempered, Marko's teenage years were spent as a hell-raising confederate of Jochen Rindt, but he calmed down sufficiently to obtain a law degree before pressing on with his own motor racing ambitions, started with Super Vee. In 1969 he drove the works McNamara in F3, but he had already tried his hand in sports cars, on which he concentrated the following year.
Driving Martini Racing's Porsche, Marko soon made his mark, finishing third at Le Mans in 1970 and then winning the classic race with Gijs van Lennep a year later. Luck generally deserted Helmut in other major events, but he handled a little Lola T212 sports car entered by Karl von Wendt to devastating effect in 1971, winning the Auvergne Trophy, the Cape 3 Hours and three rounds of the European 2-litre championship. By this time he had made a solid start to his Grand Prix career, his initial hire-drive agreement with BRM proving so satisfactory that he soon became a full team member.
For the 1972 season Marko had a BRM contract for Formula 1, and a seat in the Alfa Romeo sports car team, for whom he scored second places in both the Targa Florio and the Osterreichring 1000 Km and thirds at Daytona and in the Nurburgring 1000 Km. His F1 season started well with fourth place in the non-championship Brazilian GP, but in the French GP at Clermont Ferrand, due to cutting the corners at the Charade Circuit, a stone was thrown up by another car shattering his visor and embeding itself in the unfortunate driver's eye. Happily Marko was able to bring the car safely to a halt, but the sight of the eye could not be saved, and a potentially fine Grand Prix career was lost.
Marko subsequently stayed within the sport working for Renault Austria. He went on to run a series of racing teams and oversee several young Austrian drivers, notably Gerhard Berger and Karl Wendlinger. In recent years he has run the Red Bull Junior Team in Formula 3000 and acted as an advisor to Red Bull boss Dietrich Mateschitz when he acquired Jaguar Racing from Ford. He sold the Red Bull Junior Team in 2002.