While language dominance has been crucial in the study of bilingualism, recent research has called for more detailed measures to systematically account for the observation that bilinguals use different languages in different domains, a phenomenon formalized in the Complementary Principle. Few studies have systematically measured these language-domain relationships. Addressing these gaps, this study employs a novel, mixed-methods approach to identify the links between domain (i.e., topic) and language and to examine the effect of language dominance on language-domain relationships. Spanish-English bilinguals in the U.S. (N = 443) responded to open-ended prompts about the topics they discuss in each of their two languages and when code-switching. A qualitative thematic analysis was used to identify relevant topic categories and examine the relationships between topic and language. Subsequent analysis examined variability in language-domain mappings of bilinguals from across the language dominance continuum. Results showed that Spanish was most associated with topics of family and day-to-day life and English with academics and work. Little differentiation in language-domain mappings was found between English-dominant, Spanish-dominant, and balanced bilinguals. Findings are discussed with a focus on the methodological contributions and the implications for integrating the Complementary Principle in measures of language dominance.