Background and Objectives Current definitions of older adults’ mobility do not cover the full range of influencing factors and do not consider nursing home residents. The present study reports expert ratings from a Delphi study regarding factors influencing the mobility of long-term nursing home residents. Research Design and Methods We invited 129 professionals with expertise in a field relevant to research on mobility in older adults such as health care, medicine, or human movement science to participate in a Delphi study comprising three rounds of online questionnaires. Up to 40 experts participated in each round. In the first round, we used open-ended questions to solicit possible definitions, contextual factors, and assessment tools. In the second round the participants used Likert scales (1-10) to rate their suitability. In the final round, we presented a definition based on consensus as well as the top-rated contextual factors and assessment tools from the first 2 rounds for a final rating of agreement. Results The experts showed broad agreement on the final version of the mobility definition, with 8.9 ± 1.4 (mean ± standard deviation) out of 10 points. The experts also showed broad consensus on the selected contextual factors, with 8.4 ± 1.8 points to 8.9 ± 1.2 points (out of 10 points). This was also the case for the top 3 assessment tools selected from results of previous rounds, which showed an agreement of 7 or more points (out of 10 points) by 81.6% to 100% of all experts in the different categories. Discussion and Implications Given that an interdisciplinary group of experts considered various hitherto neglected contextual factors as relevant, the Delphi survey and its results imply the need for an updated interdisciplinary and holistic understanding of mobility in nursing home residents and can provide a basis for putting it into practice.