Needs assessment facilitates mental health services planning, provision, and evaluation. This study aimed to (a) validate a new instrument, the Montreal Assessment of Needs Questionnaire (MANQ), and (b) use this to assess variations and predictors of need (number and seriousness) in 297 individuals with severe mental disorders for 18 months, during implementation of the Quebec Mental Health Action Plan. MANQ internal and external validations were adequate. Variables significantly associated with need number and seriousness variations were used to build multiple linear regression models. Autonomous housing, not receiving welfare, not having consulted a health educator, higher level of help from services, Alcohol Use Disorders Identification Test total score, and social support were associated with decreasing need number and seriousness over time. Having a higher education was also associated with decreasing need number. In a reform context, the MANQ's unique ability to detect rapid improvement in patient needs has usefulness for Quebec mental health planning.