Shear force is commonly used to evaluate tenderness, one of the most crucial eating quality aspects of sheep meat. The effect size of various factors on tenderness is still unknown. Studies have suggested that both genetic and environmental factors contribute to the variation in meat tenderness, and there are possible interactions between these factors. An extensive data set (n = 23,696) was analyzed to examine genetic and non-genetic influences on the shear force at 5 days postmortem (SF5). SF5 was measured on lamb loins (Longissimus lumborum) taken from lambs reared over 12 years at eight sites across Australia. The results showed that all carcass traits had a significant (p < 0.001) impact on SF5, with the largest effect on SF5 associated with intramuscular fat (IMF %) (f = 1035). There was also a significant effect of sex, cold shortening at 18 ◦C, sire type and cohort on SF5 (p < 0.001), with a large variation observed between the minimum cohort at 15.9 ± 1.5 N and maximum at 51.2 ± 2.1 N. In conclusion, a complex matrix of production, processing and genetic factors impact lamb tenderness as measured by shear force. This experiment helps identify the size of the contribution of these factors towards lamb tenderness, enabling the sheep industry to enhance consumers’ satisfaction.