Context Dietary strategies may influence the fatty acid composition of sheep meat and ultimately its nutritional and sensory quality. Aim This study investigated the effects of silage to concentrate ratio and feeding duration on the concentration of intramuscular fatty acids and branched chain fatty acid in subcutaneous adipose tissue of lambs. Methods Pasture fed lambs were assigned in groups of 11 to one of nine dietary treatments consisting of silage to cereal concentrate at ratios (dry matter basis) of 100:0 (silage), 50:50 (silage:concentrate) or 0:100 (concentrate) for three feeding durations (36, 54 or 72 days) pre-slaughter. A 10th group of 11 lambs remained at pasture until slaughter. Fatty acid methyl esters were prepared using a microwave-assisted method and quantified by gas chromatography with flame ionisation detection. Results The concentration of n-3 polyunsaturated fatty acids (PUFA) and proportions of C18:3n-3 and C18:2 cis-9 trans-11 decreased for mixed feed and/or concentrate treatments, but not for silage, from 36 to 72 days. The C18:2n-6 and n-6 PUFA concentrations increased for mixed feed and concentrate, but not for silage, from 36 to 72 days. The animals that remained at pasture had lower C18:2n-6 and n-6 PUFA than those receiving mixed feed for 72 days, concentrate for 54 days and concentrate for 72 days, but higher C18:3n-3, C18:2 cis-9 trans-11 and n-3 PUFA concentrations than the indoor finishing groups. The n-6/n-3 PUFA ratio was lower in pastured lambs compared to the other groups. 4-Methyloctanoic acid and 4-ethyloctanoic acid were higher in concentrate and mixed feed treatments than in silage while 4-methylnonanoic acid increased with increase in concentrate level in the diet. Conclusion The ratio of silage to concentrate in the diet and duration of feeding pre-slaughter influence the fatty acid composition of muscle and subcutaneous adipose tissue of lambs. Implications The results are important for lamb producers in understanding the nutritional and sensory implications of altering lamb diets.