From its beginning in 1769 San Diego was a borderland entity, a place where two or more cultural and national identities met. Mexico’s independence in 1821 began the development of its national identity. From the end of the U.S.-Mexico War in 1848 to Alonzo Horton’s arrival with his San Francisco money in 1868, which shifted growth to New Town, Old San Diego remained the center of public life and retained many of its Mexican characteristics as a true borderland town. By looking at the expressed identities of individuals who made San Diego home during this period, this thesis analyzes how the convergence of national ideas meld with these citizens’ social identities to form borderland identities. To understand the construction of border identities the body chapters of this thesis focus on three aspects of identity: social identity, Mexican national identity, and U.S. national identity. Social identity is operationalized to define the specific characteristics used to analyze individuals throughout this study. While social and national identity theory are the primary tools used in the analysis of these subject citizens, this work is informed by borderland theory to expose the variety of identities manifested along the spectrum of antagonistic to synergistic and explains how border, borderland, and transborder citizens were formed. The identities of early San Diegans were multilayered constructions of the still forming Mexican and American national identities on top of the social identity complex passed to them from generations of ancestors, friends, and government. To provide insight into the development and expression of these individuals’ identities a close reading and analysis of diaries, memoirs, letters, and contemporary newspapers; data found in baptismal, marriage, death, tax assessment, and census records; and official survey maps uncover individual and family associations of class, vocation, gender, ethnicity, and heritage. What is found in this thesis is that early American San Diego was not only a geographical or national borderland, but also an identity borderland, where its citizens made social and national identity choices that helped them navigate the complex international and intercultural relationships that determined their place in that borderland.