Clinical trials to improve day-to-day function in Huntington disease (HD) require accurate outcome measures. The DSM-5 recommends the World Health Organization Disability Assessment Schedule (WHODAS) 2.0 for use in neuropsychiatric disorders. The DSM-5 also states proxy measures may be useful when cognitive function may be impaired. We tested WHODAS participant and companion ratings for differences in baseline and longitudinal function in three prodromal HD groups and a control group. Participants with prodromal HD were stratified by disease progression (low, medium, and high disease burden) based on their cytosine-adenine-guanine (CAG)-age product (CAP) score. Participant (N=726) and companion (N=630) WHODAS scores were examined for group differences, and for participant versus companion differences using linear mixed effects regression and Akaike's information criterion to test model fit. We also compared WHODAS with the Total Functional Capacity (TFC) scale. At baseline, functioning on the WHODAS was rated worse by participants in the high group and companions compared with controls. For longitudinal changes, companions reported functional decline over time in the medium and high groups. In simultaneous analysis, participant and companion longitudinal trajectories showed divergence in the high group, suggesting reduced validity of self-report. The WHODAS showed greater longitudinal difference than the TFC in the medium group relative to controls, whereas the TFC showed greater longitudinal difference than WHODAS in the high group. Results suggest the WHODAS can identify baseline and longitudinal differences in prodromal HD and may be useful in HD clinical trials. Companions may provide more accurate data as the disease progresses.