While crossing of specialized sire and dam breeds can be biologically efficient overall, a by-product is produced in the form of (crossbred) males of the dam line that are unwanted for breeding. In the UK sheep industry, most commercial females are first crosses between males of ‘longwool’ crossing sire breeds, predominantly the Bluefaced Leicester, and females of hill breeds. Genetic improvement of carcass quality in longwool breeds would benefit the surplus F1 males as well as filter through to the terminal sire cross lambs produced by the F1 ewes. As a first step, this paper aims to describe the growth and carcass characteristics of crossbred ‘Mule’ wether offspring of Bluefaced Leicester sires and Scottish Blackface and Hardy Speckled Face dams, and relate the crossbred performance to the ‘lean’ index and live conformation score of the sires and the breed of the dams. In each of 3 years, 1500 hill ewes divided over three sites were mated to 15 ram lambs, selected using an elliptical design to best represent the full spectrum of lean index (designed to improve muscle and decrease fat content while keeping live weight unchanged) and live conformation score present in the Penglas Bluefaced Leicester Group Breeding Scheme. Full growth and slaughter records were available on 2192 Mule wether lambs slaughtered after reaching finished condition, defined as the borderline between MLC fat class 2 and 3L. Measurements included live weight, ultrasonic muscle and fat depth, live conformation score, carcass hot and cold weight, MLC fat score (on the standard seven-point scale) and conformation score (EUROP and 15-point scale) and a visual estimation of subcutaneous fat cover. Nearly 800 carcasses were dissected; 20% had a full side and 80% a shoulder dissection. In part-dissected carcasses, shoulder dissection results were used to predict the fat and lean content of the other joints. Fat and lean percentages and absolute weights were then regressed on the lean index and the residual live conformation score of the sire. At the same finished condition, Scottish Blackface offspring were heavier, older, and had less fat and more lean based both on ultrasound and on carcass dissection measurements compared with Hardy Speckled Face offspring. The index score of the sire had a positive effect on the lean content and a negative effect on fat content of all joints. Conformation of the sire did not have a significant effect on any of the carcass composition measurements, but most conformation traits measured on the animal itself (live or on the carcass) were positively related to fat and negatively to lean content in the carcass. When taking into account differences in visually assessed subcutaneous fat percentage, the carcass conformation traits persisted only in having a positive effect on carcass fat content; none of the conformation traits had an effect on carcass lean content. The results show that selection of sires on lean index is an effective way to improve carcass composition in Mule wethers, but selection on conformation is ineffective.