Background: Muscle mass, strength and function declines with advancing age. Strength training (ST) improves these parameters in older adults, but the gains often disappear after completion of a short-term intervention. The purpose of the present study was to investigate muscle mass, -strength and -function one year after the completion of a successful long-term (12 months) supervised ST program in older adults. Method: Men and women (n = 419, age: 62–70 years) completed one year of supervised heavy resistance training (HRT, n = 143) or moderate intensity resistance training (MIT, n = 144) and were compared to a non-exercising control group (CON, n = 132). At 1-year follow-up, 398 participants returned for measurements of muscle power, -strength and -mass, physical function, body composition, hippocampus volume and physical/mental well-being. The results were compared to pre-training (baseline) and post-training (1-year) values. Further, the participants from the two previous training groups (HRT + MIT, n = 265) were divided into 1) those who on their own continued the ST program (>9 months) the year after completion of the supervised ST program (CONTIN, n = 65) and 2) those who stopped during the follow-up year (<9 months) (STOP, n = 200). Results: Out of all the improvements obtained after the 1-year training intervention, only knee extensor muscle strength in HRT was preserved at 1-year follow-up (p < 0.0001), where muscle strength was 7% higher than baseline. Additionally, the decrease in muscle strength over the second year was lower in CONTIN than in STOP with decreases of 1% and 6%, respectively (p < 0.05). Only in CONTIN was the muscle strength still higher at 1-year follow-up compared with baseline with a 14% increase (p < 0.0001). The heavy strength training induced increase in whole-body lean mass was erased at 1-year follow-up. However, there was a tendency for maintenance of the cross-sectional area of m. vastus lateralis from baseline to 1-year follow-up in HRT compared with CON (p = 0.06). Waist circumference decreased further over the second year in CONTIN, whereas it increased in STOP (p < 0.05). Conclusion: Even though long-term strength training effectively improved muscle function and other health parameters in older adults, only knee extensor muscle strength was preserved one year after completion of heavy (but not moderate intensity) resistance training. Continuation of strength training resulted in better maintenance of muscle strength and health, which indicates that it is required to continue with physical activity to benefit from the long-term effects of strength training upon muscle function and health in older men and women.