Heart transplant allocation is primarily focused on preventing waitlist mortality. To prevent organ wastage, future allocation will need to balance risk of waitlist mortality with post-transplant mortality. To understand the complex factors affecting post-heart transplant survival and to determine the importance of possible sex-specific risk factors, we studied data from all adults in the Scientific Registry of Transplant Recipients (SRTR) database who underwent heart transplant between January 1, 2004 and July 1, 2018. We identified three phases affecting survival: early post-operative, constant, and a late phase, and found that the factors and interactions predicting post-transplant mortality varied with the three phases. Machine learning methods and conventional statistical methods were both important in this study and provided complementary information to better understand post-transplant mortality.