This diachronic study is a variationist analysis of subject pronoun expression (SPE) in two key Spanish-language Arizonan newspapers, El Fronterizo (1878–1914) and El Tucsonense (1915–1957), following Tucson’s annexation to the United States through the Gadsden Purchase, a period of great social change during which the Spanish-speaking population in the city underwent a gradual process of anglicization. Since some research on SPE in Spanish in the United States suggests that English-Spanish bilingualism increases the use of overt subject personal pronouns (SPPs) because of their almost categorical use in English, this study’s main aim is to track the initial stages of such progression in a period when social bilingualism was steadily extending in Tucson. In this respect, our results show that the presence of overt SPPs does increase over time in the data analyzed; however, lower rates of overt SPPs in contemporary Tucson and Phoenix spoken Spanish raise the possibility that the percentage surge in the aforementioned period is rather due to the offline written nature of the newspapers, which, for instance, weakens the effect of online constraints, such as switch reference, ambiguous TAM endings, and non-reflexive verbs. Even so, regression analyses with the mixed-effects statistical software Rbrul reveal that the linguistic factor groups shaping SPE in the diachronic data are essentially the same ones found operating in contemporary varieties of Spanish.