Allen founded Vulcan Inc., a private investment firm, to manage his business and philanthropic activities. Vulcan served as the umbrella for his diverse portfolio of investments and projects.

Allen started Asymetrix corporation in Bellevue, Washington to develop software applications.

Allen invested heavily in Charter Communications, a cable television company, and played a key role in its growth and expansion. He served as chairman of the board for several years.

Allen invested heavily in real estate, particularly in Seattle's South Lake Union neighborhood. His investments transformed the area into a hub for technology companies and research institutions. He became a major landowner and developer in the city.

Allen pursued a variety of other business ventures, including investments in technology companies, media properties, and entertainment projects. He was involved in the development of the Experience Music Project (EMP) Museum (later renamed the Museum of Pop Culture - MoPOP) and invested in space exploration technologies.