Background: There is a need for validated, standardised tools for the assessment of executive functions in adults with intellectual disabilities (ID). This study examines the validity of a test of planning and problem solving (Tower of London) with adults with ID. Method: Participants completed an adapted version of the Tower of London (ToL) while day-centre staff completed adaptive function (Adaptive Behaviour Scale--Residential and Community: Second Edition, modified version) and dysexecutive function (DEX-Independent Rater) questionnaires for each participant. Correlation analyses of test and questionnaire variables were undertaken. Results: The adapted ToL has a robust structure and shows significant associations with independent living skills, challenging behaviour and behaviours related to dysexecutive function. Conclusions: The adapted ToL is a valid test for use with people with ID. However, there is also a need to develop other ecologically valid tools based on everyday planning tasks undertaken by people with ID.