Social isolation, given its established association with mortality, and risk of poor physical and/or mental health over the life course, has attracted the attention of researchers. However, such measures have been highly variable and often lack conceptual clarity. This study, which drew on a conceptual framework for classifying social isolation measures, provides a rationale for the development and validation of a composite Social Isolation Index (SII) using the Canadian Longitudinal Study on Aging (CLSA), together abbreviated as CLSA-SII. CLSA-SII was developed using standard methodology for developing other measures, including the multimorbidity resilience and successful aging indices. Associations of the CLSA-SII and four key outcome variables (life satisfaction scale, depression, perceived mental health, and perceived health) were performed using regression analyses. Associations between the CLSA-SII index and the four outcome variables resulted in statistically significant moderate associations in the expected direction, and are consistent with prior research, demonstrating good concurrent validity.