The main objective of the research was to compare the cognitive functioning of a sample of persons with IDD (Intellectual and Developmental Disabilities) before the pandemic (2019) and after the pandemic (2020 and 2021), and to analyse the impact according to age and level of IDD impairment. The participants were 92 persons with IDD, of whom 43 were female (46.7%). The mean age in 2019 was 47.07 years (SD = 6.78). All the participants were living with family members. The CAMCOG-DS test from CAMDEX was used to assess the cognitive functions. The results indicate a worsening in cognitive functions (attention–concentration, abstract thought, language, and praxis) after lockdown, in both the total group of participants and the mild–moderate impairment group, and in both age groups. In the severely affected group, we found an improvement in the cognitive functions assessed after lockdown. These results are similar to those found in people with dementia and in the general ageing population. Results were discussed in relation to the consequences of isolation in people with IDD, as well as providing guidelines for future pandemic situations.