The chemical composition and volatile profile of the Longissimus thoracis et lumborum muscle of lambs reared in two distinct production systems (intensive and extensive) was evaluated. For this, sixty-six lambs for meat production of the autochthonous Gallega Iberian breed were raised with concentrate and grass in intensive (30 animals) and extensive (36 animals) system, respectively, until 4–4.5 months of age when they were slaughtered. Subsequently, Longissimus thoracis et lumborum muscles were excised for the determination of chemical composition (moisture, intramuscular fat, protein, and ash percentages) and volatile substances. The aftermaths obtained evidenced that moisture (75.90% and 75.68%), intramuscular fat (1.68% and 1.76%), and protein (20.62% and 20.97%) contents were not significantly (P > 0.05) affected by the production system. However, the extensively-fed lambs displayed a higher ash content (1.35% vs. 1.24%). Additionally, the total volatile content was also not significantly (P > 0.05) influenced by feeding system. Despite this, the total content of hydrocarbons, acids, aldehydes, ketones, esters, ethers, furans, sulfur compounds, and others was significantly (P < 0.05) affected by diet, being the alcohol family the only group not influenced (P > 0.05) by the production system (1321.3 vs. 1211.3 AU × 104/g fresh muscle). Specifically, intensively-fed lambs showed significantly higher amounts for all volatile families apart from ketones (2215 vs. 2826 AU × 104/g fresh muscle) and sulfur compounds (22.7 vs. 123.7 AU × 104/g fresh muscle). In addition, benzyl alcohol and carbon disulfide were proposed as appropriate biomarkers for grass diets, while 1-butanone, 2-heptanone, and furan, 2-penthyl were indicated as suitable tracers for concentrate-based diets.