In 1928, Purcell was appointed by Governor C. C. Young as the California State Highway Engineer. In 1929, he was appointed technical adviser to the 1929 Hoover-Young San Francisco-Oakland Bay Bridge Commission, which commissioned construction of the -long San Francisco–Oakland Bay Bridge. When the commission finished its work, Purcell was appointed by Governor Jim Rolph as the Chief Engineer responsible for the design and construction of the bridge. During construction, the project was held up by the maritime strikes of 1934. Despite these and many other challenges, the bridge was completed several months ahead of schedule. The western portion of the bridge relied on a central -tall anchorage, of which is beneath the surface of the water. It is the world's largest pier. The tunnel through Yerba Buena Island was at the time the largest bore tunnel in the world. Its completion was the realization of dreams first conceived by local citizens in 1850. Upon its completion, it was the longest and at $77,600,000, the most expensive bridge ever built. More than 6,500 employees worked on the project.