Ertl was born in Zell am See. He was in the same school as Jochen Rindt and Helmut Marko, so he had an early relation to motorsports. In 1969 he bought a Formula V, won six races, but also rolled at the Nürburgring. He was second in the European Cup with a Kaimann chassis in 1970, and started also in Formula 3 with a March Engineering 703.
In 1971 he moved to Touring car racing, driving an Alfa Romeo in the Deutsche Rennsport Meisterschaft (DRM) and the European Touring Car Championship.
From 1974 to 1976, Ertl raced in Formula 2, eg. the ADAC Eifelrennen.
In 1975, sponsorship by Warsteiner Brauerei had allowed him to drive in F1 with a Hesketh in their golden livery. In his debut at the 1975 German Grand Prix he finished 8th, retired in the next GP, to finish 9th at Monza.
Encouraged by the results, a full season with Hesketh was planned for 1976. In the 1976 South African Grand Prix he qualified in the last row and finished 15th. The next races saw him either not qualify at all, or at the end of the grid, with the car failing soon. Despite not being satisfied with the value the team gave him for the money, he almost scored a point when coming in 7th at the 1976 British Grand Prix, 3 laps down though.
Two weeks later at the 1976 German Grand Prix Ertl was one of the four drivers who helped pull Niki Lauda from his burning Ferrari after Lauda's infamous crash during the second lap of the race. The rest of the season saw two other 8th places as highlights, at home and at the wet 1976 Japanese Grand Prix.
Ertl continued with Hesketh for some European races, scoring 9th at the 1977 Belgian Grand Prix, but quit after the 1977 French Grand Prix where he failed to qualify once again.
For a few races in 1978, Ertl entered with Ensign, but things were even worse there, with the cars failing to finish or even to pre-qualify, like in the 1978 Italian Grand Prix. He got a chance on the spare car of German Team ATS but couldn't qualify that either.
Ertl tried once again with ATS in the 1980 German Grand Prix with the same result, so the 1978 Austrian Grand Prix in front of his home crowd was the last GP he actually took part in.
Meanwhile based on experience with their BMW, Schnitzer Motorsport developed a Toyota Celica Turbo for Ertl in 1977. As his career highlight, Harald Ertl won the DRM in 1978 driving a BMW 320i Turbo for Schnitzer Motorsport.
In 1979 and 1980, he drove successfully for Zakspeed, winning several races with their Ford Capri turbo. They also developed a mid-engined Lotus Europa for the 1000km Nürburgring. Harald Ertl did not race in 1981, but planned a return for the 1982 Renault 5 Turbo Cup.
Like Graham Hill and other pilots before, Ertl was killed in a general aviation aircraft accident at only 33 years old, whilst flying his family to their holiday home in Sylt in Northern Germany. His wife and son survived with injuries.