Countries and cities around the world are searching for ways to plan urban development to accommodate the growing demand for public infrastructure and amenities due to high rates of urbanization. Urban policies deemed to be successful are often adopted by other cities and hence applied across contexts. Urban Policy Mobility (UPM) theories study the ways in which political relationships, power dynamics, and other potential drivers influence the circulation of urban policies. UPM provides a guiding framework to analyze the policymaking associated with mobilized urban policies to understand the socio-spatial processes that motivate the adoption of urban policies from elsewhere. This dissertation engages with the UPM literature by investigating the assemblage and implementation processes of a mobilized urban policy to explore how the fixed socio-spatial processes embedded within a particular planning culture interact with and absorb a flowing urban policy. The term flowing in this study refers to the process of transposing policies from one place to another. Little attention has been paid to how the planning culture in a developing country can shape the adoption (or non-adoption) of certain policy elements, and what challenges arise during the implementation of a flowing urban policy. Given this context, this study answers two research questions: (a) How does urban policy mobility unpack in practice in response to local socio-spatial processes? and (b) What challenges emerge when policies are transposed into new urban policy environments that are dissimilar from those in which they originated? A case study methodology was used to study the changes that occurred when a Transit-Oriented Development (TOD) urban policy was introduced in Riyadh, Saudi Arabia. A qualitative content analysis of archival resources, documents, and semi-structured interviews revealed how local urban planning conditions and challenges influenced Riyadh's TOD assemblage and implementation. Public officials and consultants from Riyadh, national officials, and experts were interviewed to develop a clear understanding of the TOD policy assemblage and implementation processes. The two main findings from this research are that: 1) there was an incomplete translation of TOD into the local planning culture, primarily due to Riyadh's stronger emphasis on the density, diversity, and design features of TOD and less concern with the sustainability dimensions; and 2) several governance and prioritization challenges emerged during the policy implementation process, which stem from institutional constraints and institutional and resource gaps. This research expands the UPM field by tracing the trajectory of policy mutation due to local socio-spatial processes. Additionally, this study provides a conceptual framework that synthesizes three heterogeneous elements: planning culture, planning policy, and policy carriers. It offers a methodological contribution that advances UPM analysis to better explain policy mutation. This study can be used as a cautionary tale for officials engaged in adopting urban policies that originate in other jurisdictions. / Doctor of Philosophy / City officials around the world are looking beyond their borders for urban policies that can promote sustainability and improve quality of life. However, those officials rarely consider how differences between urban areas can alter the nature of policies being adopted. To address this challenge, the field of Urban Policy Mobility (UPM) emerged to shed light on how the unique local factors that shape each city environment affect what elements of an urban policy are and are not adopted. This study examines the changes that occurred when a Transit-Oriented Development (TOD) urban policy was introduced in Riyadh, Saudi Arabia. In order to learn about the changes and challenges associated with Riyadh's TOD policy, interviews were conducted with employees responsible for the policy. Additionally, TOD policy documents and other publications that contained information about Riyadh's TOD were reviewed to build a deeper understanding of why certain policy elements were implemented and others were not. This study found that: 1) the TOD policy was not completely translated into Riyadh, with greater emphasis placed on increasing building density, diversifying land uses, and enhancing design aesthetics, and less on other important policy elements that enhance economic and social sustainability; and 2) the implementation of the TOD policy led to governance challenges due to the differences in how urban planning is undertaken in Riyadh compared to western countries. This in-depth study of Riyadh's experience can inform other cities that are looking to implement urban policies borrowed from other countries.