Over the past decades, life expectancy in Hong Kong has been increasing steadily. However, the gender gap in life expectancy has experienced a comparatively high level of plateauing state. Based on data from the Human Mortality Database and the official statistics, an age- and cause-decomposition method was used to analyze contributions of main age groups and major causes of death to the gender gap over the period 1986–2016. The results showed that a female-advantage in the mortality among those aged 25 years and above was mainly responsible for the male–female gap in life expectancy in Hong Kong. In consideration of major causes of death, lower female mortality from non-communicable diseases, particularly malignant neoplasms, and pneumonia at older age groups, was the major contributor to the female-favored life expectancy. Public efforts to reduce excess mortality rates from non-communicable diseases and pneumonia among older male population may help narrow the gender gap in life expectancy in Hong Kong.