Since its foundation in 1904, the Mount Wilson Observatory has been at the centre of the development of astrophysics. Perched atop a mountain wilderness, two mammoth solar tower telescopes and the 60- and 100-inch behemoth night-time reflectors, were all the largest in the world. Research has centred around two main themes - the evolution of stars, and the development of the universe. This first volume in a series of five histories of the Carnegie Institution describes the people and events, the challenges and successes that the Observatory has witnessed. It includes biographical sketches of forty of the most famous Mount Wilson pioneer astronomers working during the first half of the twentieth century. Contemporary photographs illustrate the development and use of some of the innovative instruments that filled the observatory during this time. This story brings together the elements that formed modern theories of stellar evolution and cosmology.