The cooking time and internal temperature for lamb depend on how you like to serve your lamb. - Medium rare  - between 130 F to 135 F, approximately 20 minutes per pound - Medium - between 135 F to 140 F, approximately 22 to 23 minutes per pound - and Medium well - between 145 F to 150 F, approximately 25 minutes per pound. - Well Done is between 155 to 165 F, or approximately 30 minutes per pound.