Purpose: The study examined the longitudinal interplay of anthropometric, metabolic, and neuromuscular development related to performance in adolescent national-level swimmers over 12 months. Methods: Seven male and 12 female swimmers (14.8 [1.3] y, FINA [International Swimming Federation] points 716 [51]) were tested before (T0) and after the preparation period (T1), at the season’s peak (T2), and before the next season (T3). Anthropometric (eg, fat percentage) and neuromuscular parameters (squat and bench-press load-velocity profile) were assessed on dry land. Metabolic (cost of swimming [C], maximal oxygen uptake [$\dot{\mathrm{V}}{\mathrm{O}}_2\text{peak}$], and peak blood lactate [bLa_peak]) and performance (sprinting speed [v_sprint] and lactate thresholds [LT1 and 2]) factors were determined using a 500-m submaximal, 200-m all-out, 20-second sprint, and incremental test (+0.03 m·s⁻¹, 3 min), respectively, in front-crawl swimming. Results: v_sprint (+0.6%) and LT1 and 2 (+1.9–2.4%) increased trivially and slightly, respectively, from T0 to T2 following small to moderate strength increases (≥+10.2%) from T0 to T1 and $\dot{\mathrm{V}}{\mathrm{O}}_2\text{peak}$ (+6.0%) from T1 to T2. Bench-press maximal strength and peak power correlated with v_sprint from T0 to T2 (r ≥ .54, P < .05) and LT2 at T1 (r ≥ .47, P < .05). Changes in fat percentage and $\dot{\mathrm{V}}{\mathrm{O}}_2\text{peak}$ (T2–T1 and T3–T2, r ≤ −.67, P < .01) and C and LT2 (T2–T0, r = −.52, P = .047) were also correlated. Conclusions: Increases in strength and $\dot{\mathrm{V}}{\mathrm{O}}_2\text{peak}$ from preparation to the competition period resulted in improved sprint and endurance performance. Across the season, upper-body strength was associated with v_sprint and LT2, although their changes were unrelated.