Objective: As myotonic dystrophy type 1(DM1) evolves slowly and interventional trials often have a short duration, responsive outcomes in DM1 are needed. The objective of this study was to determine the responsiveness of muscle strength, balance, and functional mobility measurements after a 1-year follow-up period in individuals with DM1. Methods: Sixty-three adults with noncongenital DM1 completed the following assessments at baseline and at 1-year follow-up: Handheld dynamometry (lower limbs), stationary dynamometry (lower limbs), step test, timed-up-and-go test (TUG), modified clinical test of sensory integration and balance (mCTSIB), feet-together stance, tandem stance, one-leg stance, 10-meter walk test, and sit-to-stand test. Results: Change was captured by stationary dynamometry (proximal flexor and extensor muscles), handheld dynamometry (proximal flexor and distal extensor muscles), TUG, and mCTSIB (P ≤ 0.04). Ceiling or floor effects were shown for most static balance tests. Interpretation: Overall, adequate responsiveness was shown for both muscle strength dynamometers, TUG and mCTSIB. These outcomes are therefore likely candidate endpoints for clinical trials lasting 1 year. Most static balance tests are not responsive and not recommended in a heterogeneous DM1 population.