We devised a new Functional Mobility Scale (FMS) to describe functional mobility in children with cerebral palsy, as an aid to communication between orthopaedic surgeons and health professionals. The unique feature of the FMS is the freedom to score functional mobility over three distinct distances, chosen to represent mobility in the home, at school and in the wider community. We examined the construct, content, and concurrent validity of the FMS in a cohort of 310 children with cerebral palsy by comparing the FMS to existing scales and to instrumented measures of physical function. We demonstrated the scale to be both valid and reliable in a consecutive population sample of 310 children with cerebral palsy seen in our tertiary referral center. The FMS was useful for discriminating between large groups of children with varying levels of disabilities and functional mobility and sensitive to detect change after operative intervention.