© 2018 Elsevier Inc. Aims: To determine the association between 20-year trajectories in insulin resistance (IR) since young adulthood and appendicular lean mass (ALM) at middle-age in adults without diabetes. Methods: A prospective cohort study was designed among young and middle-aged US men (n = 925) and women (n = 1193). Fasting serum glucose and insulin were measured five times in 1985–2005. IR was determined using the homeostasis model assessment (HOMA). ALM was measured in 2005 and ALM adjusted for BMI (ALM/BMI) was the outcome. Sex-specific analyses were performed. Results: Three HOMA-IR trajectories were identified. Compared to the low-stable group, the adjusted ALM/BMI difference was −0.041 (95% CI: −0.060 to −0.022) and −0.114 (−0.141 to −0.086) in men, and −0.052 (−0.065 to −0.039) and −0.043 (−0.063 to −0.023) in women, respectively, for the medium-increase and high-increase groups. Further adjusting for the treadmill test duration attenuated these estimates to −0.022 (−0.040 to −0.004) and −0.061 (−0.089 to −0.034) in men and −0.026 (−0.038 to −0.014) and −0.007 (−0.026 to 0.012) in women. Conclusions: Compared to the low-stable insulin resistance trajectory between early and middle adulthood, the high-increase trajectory was associated with lower ALM/BMI in middle-aged men, but not women, without diabetes, after adjusting for cardiorespiratory fitness.