Sarcopenia and frailty are age-related syndromes with negative effects on the quality of life of older people and on public health costs. Although extensive research has been carried out on the effects of physical exercise and physical syndromes, there is a knowledge gap when it comes to the effect of resistance training on muscular strength, physical performance, and body composition at early (prevention) and late (treatment) stages in both syndromes combined. We conducted this systematic review and meta-analysis (CRD42019138253) to gather the evidence of randomized controlled trials examining the effects of resistance training programs lasting ≥8 weeks on strength, physical function, and body composition of adults ≥65 years old diagnosed with pre-sarcopenia, sarcopenia, pre-frailty, or frailty. A search from the earliest record up to and including December 2020 was carried out using the PubMed, Scopus, Web of Science, and Cochrane Library databases. A total of 25 studies ( = 2267 participants) were included. Meta-analysis showed significant changes in favour of resistance training for handgrip (ES = 0.51, = 0.001) and lower-limb strength (ES = 0.93, < 0.001), agility (ES = 0.78, = 0.003), gait speed (ES = 0.75, < 0.001), postural stability (ES = 0.68, = 0.007), functional performance (ES = 0.76, < 0.001), fat mass (ES = 0.41, = 0.001), and muscle mass (ES = 0.29, = 0.002). Resistance training during early stages had positive effects in all variables during early stages (ES > 0.12), being particularly effective in improving gait speed (ES = 0.63, = 0.016) and functional strength (ES = 0.53, = 0.011). Based on these results, resistance training should be considered as a highly effective preventive strategy to delay and attenuate the negative effects of sarcopenia and frailty in both early and late stages.