Loyalty

• During the final mission, the game will check the loyalty of each of your team members when determining whether they will live or die. To ensure Mordin's loyalty, you must have completed his loyalty mission, called "Mordin: Old Blood." If you have not completed this mission, Mordin may die during the final mission no matter what choices you make.
  
  

Wrong Assignments

• At several points during the final mission, you need to assign a team member to perform a special task. Mordin is unsuitable for most of these tasks. If you send him through the vents to open the doors or choose him as the leader of the diversion team, he will be killed. Additionally, if you keep Mordin in your active team and select a Biotic expert who is not loyal to you and is not Samara, Morinth or Jack, Mordin may be killed. Lastly, assigning Mordin to escort the crew back to the ship will cause him to die if you have not completed his loyalty mission.
  
  

Final Battle

• Once you reach the final battle, you will need to choose which of your team members will be on the offensive and which will be left behind to defend. Once you have chosen the members of your offensive team, the game will calculate the total defensive strength of the team you chose to leave behind; if this defensive strength is not high enough, the game may kill one or all the team members you left behind. Mordin has low defense; he may be killed if left behind, especially if you also leave behind other team members with low defense, such as Jack and Tali.
  
  

Saving Mordin

• If you have not completed Mordin's loyalty mission, always keeping him in your active squad has the highest chance of guaranteeing his survival. If Mordin is loyal to you, he is unlikely to die unless assigned to the wrong task. If you do not wish to have him in your active squad during the final battle, assigning him to escort the crew members back to the ship will ensure his survival.