(1) Background: Patient safety culture (PSC) encompasses the values, attitudes, norms, beliefs, practices, perceptions, competencies, policies, and behaviours of professionals that determine organisational commitment to quality and patient safety. Few studies use mixed methods to analyse patient safety culture, and none offer the richness of using a mixed methodology to develop their theoretical model. This study aims to identify the factors nurses believe contextualise and influence PSC in relation to existing theoretical frameworks. (2) Methods: This study employed a sequential explanatory mixed-methods design combined with the Pillar Integration Process for data integration. (3) Results: In the final data integration process, 26 factors affecting nurses' PSC were identified. Factors nurses related to PSC not being assessed with the tool used in phase 1 were notification system, flow of patients, patient involvement, resources and infrastructure, and service characteristics. (4) Conclusions: This mixed-methods study provides an opportunity to identify the weaknesses and strengths of currently developed theoretical frameworks related to PSC and offers content for its improvement. Even though multiple studies aim to assess PSC using existing quantitative method tools, the development of this study offers a glimpse of some aspects relevant to nurses' PSC not included in the theoretical framework of the said tools, such as patient involvement, the flow of patients, and service infrastructure.