In March 1910, a young restaurateur called Louis Bush refurbished an old Mack Truck chassis, painted it blue and gray and began offering sightseeing tours around the city of Washington, D.C. By 1926, Gray Line had expanded to other booming cities including New York, Chicago, Detroit, New Orleans, Los Angeles, San Francisco, as well as internationally to Toronto and Havana.

With peacetime following World War II, Harry J. Dooley, a former Gray Line employee, acquired the company and helped re-establish Gray Line Chicago.

Dooley soon became president of Gray Line and is today considered the father of the sightseeing industry.

With the advent of jet travel and increased gateways in the late 1940s and early 1950s, Dooley expanded Gray Line's market throughout the United States and established Gray Line companies in Canada, Mexico, Hawaii and Puerto Rico.

Gray Line has a presence in over 700 locations, spanning six continents, and is the largest sightseeing company in the world.