After studying medicine, Udo Müller founded the advertising agency Lunenburg & Partner in 1985 and entered the outdoor advertising business in 1987. In 1990, he founded Ströer City Marketing GmbH with Heinz W. Ströer. After the company was renamed Ströer Out-of-Home Media AG in 2002, Müller took it public in 2010. Under his leadership, the Ströer Group developed into one of the leading outdoor advertisers and expanded its business to include online marketing and dialogue media. Ströer is also a publisher of media brands such as T-Online and Watson.de. Udo Müller is now the largest single shareholder and co-CEO of Ströer. Udo Müller is also involved in other companies, particularly in Cologne's start-up scene, through his investments in private companies such as Planche GmbH, PepperPeaches GmbH, Atlanta Beteiligungen Verwaltungs-GmbH and APM Verwaltungs GmbH.