Background: Given the much greater COVID-19 mortality risk experienced by people with intellectual disabilities (ID), understanding the willingness of people with ID to take a COVID-19 vaccine is a major public health issue. Method: In December 2020 to February 2021, across the United Kingdom, 621 adults with ID were interviewed remotely and 348 family carers or support workers of adults with ID with greater needs completed an online survey, including a question on willingness to take a COVID-19 vaccine if offered. Results: Eighty-seven per cent of interviewees with ID were willing to take a COVID-19 vaccine, with willingness associated with white ethnicity, having already had a flu vaccine, gaining information about COVID-19 from television but not from social media, and knowing COVID-19 social restrictions rules. A percentage of 81.7% of surveyed carers of adults with ID with greater needs reported that the person would be willing to take a COVID-19 vaccine, with willingness associated with white ethnicity, having a health condition of concern in the context of COVID-19, having had a flu vaccine, being close to someone who had died due to COVID-19, and having shielded at some point during the pandemic. Conclusions: Reported willingness to take the COVID-19 vaccine is high among adults with ID in the United Kingdom, with factors associated with willingness having clear implications for public health policy and practice.