Using a meat thermometer is the most accurate way to determine the doneness of lamb. Insert the thermometer into the thickest part of the meat, away from bones and fat. Rare: 125-130°F (51-54°C) Medium rare: 130-140°F (54-60°C) Medium: 140-150°F (60-65°C) Medium well: 150-160°F (65-71°C) Well done: 160°F (71°C) and above