This study examines how functional disability worsens among older adults exposed to extreme heat, particularly those socially isolated. Analyzing data from over 35,000 older adults aged 50 or older from the Health and Retirement Study from 1996-2018, this study found that more frequent exposure to extreme heat is associated with an increase in the number of instrumental activities of daily living (IADL) that older adults find difficulty in performing over time. This heat-related disability progression is greater among those living alone and not working. However, findings indicate that maintaining contact with children and receiving higher levels of support from friends can alleviate the risk of IADL disability progression amidst extreme heat days for older adults with limited social relationships at home and work. By examining various aspects of social isolation and their nuanced effects, this study underscores the need for social support and assistance for older adults during extreme heat.