This dissertation examines the maritime transportation networks of California and their impacts on U.S. imperialism from 1848 to 1898. It argues that the discovery of gold in the Sierra Nevada foothills in January 1848 provided a critical boost to American empire-building outside the North American continent. Sea routes to San Francisco carried American gold seekers from the Atlantic coast via ports in the Caribbean, Central and South America, and the islands of the Pacific. Maritime transportation thus created transnational sites of social interactions during the California Gold Rush (1848-c.1860), where mid-century Anglo-Americans formulated, revised, and circulated their ideas about foreign lands and people. The circuits of human movements for the riches in California became a driving force of U.S. overseas expansion, not only by creating a material foundation of imperial expansion in the U.S.-built transportation infrastructure, but also by reinforcing the cultural basis for empire as Anglo-American settlers and travelers reaffirmed and consolidated their understanding of racial superiority and territorial entitlement. The informal, sociocultural production of imperial visions and realities by these private actors helped advance American interests in the Pacific Ocean even after gold fever subsided. Post-rush movements of ex-Californians, through filibustering expeditions against Mexico and Nicaragua and Anglo-American settler migration to Hawaii, Chile, and British Columbia, constantly challenged the national boundaries and sustained visions of empire. The Pacific Mail Steamship Company, an undeniable beneficiary of California’s growth, operated as an informal agent of colonization in the Pacific from 1867 on, when the inauguration of the company’s transpacific shipping route ignited debates over seizing the islands of the Pacific to build coaling depots. At the same time, steam-powered ocean transport brought a massive number of Chinese immigrants, leading to the immigration control and delimitation of the American nation in the late-nineteenth century. In the end, this study of Gold Rush-induced transnational connections provides a window into the ways in which American people dealt with the problem of mobility and the emergence of empire during the decades leading up to 1898, when the United States became an overseas empire with the annexation of Hawaii and the occupation of the Philippines.