In 1974, Happel became CEO of GEA Group, a small family business established by his father in 1920 with approximately 100 million EUR revenues.

Over the next 25 years, he transformed GEA into a global system supplier and introduced machinery and plants for the food processing industry, power industry, air-conditioning and refrigeration industry. After making over 70 acquisitions, GEA had over 200 subsidiaries in 60 countries. In 1989 he took his company public at the Frankfurt stock exchange, but kept the majority of the voting stock in the family. By 1999, Happel had increased GEA’s revenues to over €2.6 billion, as well as its net earnings proportionally.

In 1999 he merged his controlling stake with Germany’s MG Technologies, remained the largest shareholder of the combined businesses and, once MG Technologies merged into GEA Group AG, sold his dominant stake to institutional investors in 2006. GEA Group AG is the largest listed German machinery manufacturer and is broadly diversified. In 2018, it accounted for revenues of approximately EUR 4,8 bn and employed approximately 18,600 people.