Hagen was born and raised in Norway. He earned a degree in physics at the Norwegian Institute of Technology and an MBA from Harvard Business School in 1968.
Hagen was a consultant, and later partner, for McKinsey & Company in Europe, helping to bring Holland America Line back from near-bankruptcy in 1974. In 1976, he became CEO of Bergen Steamship Company (Bergenske Dampskibsselskab), and then became CEO of Royal Viking Line from 1980 to 1984. He joined the board of Holland America Line in 1985, and later served on the board of Kloster Cruise. In the early 1990s, he acquired a 27% stake in Royal Nedlloyd, a Rotterdam-based shipping company, becoming the biggest shareholder. He sold his shares several years later at a loss.
In 1997, Hagen founded Viking River Cruises with the purchase of four river ships, launching the company with four Russian river cruises. In May 2013, the company announced plans to start an ocean cruise division, and changed its name to Viking Cruises. Its first ocean cruise was launched in 2015. As of 2017, the company has a fleet of over 60 vessels, and four ocean going ships.