Inpatient mobility programs can help older adults maintain function during hospitalization. Changing hospital practice can be complex and require engagement of various staff levels and disciplines; however, we know little about how interprofessional teams organize around implementing such interventions. Complexity science can inform approaches to understanding and improving multidisciplinary collaboration to implement clinical programs. To examine, through a complexity science lens, how clinical staff's understanding about roles in promoting inpatient mobility evolved during implementation of the STRIDE (assiSTed eaRly mobIlity for hospitalizeD older vEterans) hospital mobility program. Qualitative study using semi-structured interviews. Ninety-two clinical staff at eight Veterans Affairs hospitals. STRIDE is a supervised walking program for hospitalized older adults designed to maintain patients' mobility and function. We interviewed key staff involved in inpatient mobility efforts at each STRIDE site in pre- and post-implementation periods. Interviews elicited staff's perception of complexity-science aspects of inpatient mobility teams (e.g., roles over time, team composition). We analyzed data using complexity science-informed qualitative content analysis. We identified three key themes related to patterns of self-organization: (1) individuals outside of the "core" STRIDE team voluntarily assumed roles as STRIDE advocates, (2) leader-champions adapted their engagement level to match local implementation team needs during implementation, and (3) continued leadership support and physical therapy involvement were key factors for sustainment. Staff self-organized around implementation of a new clinical program in ways that were responsive to changing program and contextual needs. These findings demonstrate the importance of effective self-organization for clinical program implementation. Researchers and practitioners implementing clinical programs should allow for, and encourage, flexibility in staff roles in planning for implementation of a new clinical program, encourage the development of advocates, and engage leaders in program planning and sustainment efforts.