Krankl started his professional career at Rapid Wien and stayed there for 8 years, apart from a year at Wiener AC. He won the European Golden Boot at Rapid Wien in 1978, attracting the interest of Barcelona. His spell at Barcelona was successful, winning the Copa del Rey in his first season as well as the Cup Winners' Cup (scoring in the final itself), and picking up the top goalscorer's award in La Liga that season with 29 goals. He returned to Rapid Wien in 1981, captaining the side and again scoring over 100 goals for them over the next 5 years. He played a major part in the most successful of Rapid teams in the 1980s, claiming the League crown twice and most prominently losing the Cup Winners' Cup Final in 1985 against Everton. In 1986, he moved to become player-manager at Wiener Sport-Club.
Krankl was named Austrian Player of the Year a record five times, and was voted the most popular Austrian player of the last 25 years.
One of his major achievements was helping Austria qualify for the 1978 World Cup, the first time Austria had qualified for the World Cup in twenty years. He then went on to score the winning goal against West Germany in a game dubbed The miracle of Córdoba, which the Austrians won 3–2 and which was Austria's first win against West Germany in 47 years, earning Krankl legendary status in his homeland. Scored in the 88th minute, the live footage of the goal still regularly features on national television, and the audio commentary ("Tor Tor Tor Tor Tor Tor, I werd' narrisch" – "Goal goal goal goal goal goal, I'm going crazy") is immediately recognised by Austrian football fans.
Krankl has also enjoyed domestic success with his music, with several releases entering the Austrian charts. His biggest musical success was the single "Lonely Boy", released in 1985 and peaking at number 2.