Mangan was a lonely and often difficult man who suffered from mood swings, depression and irrational fears, and became a heavy drinker and opium user. His appearance grew eccentric, and he was described by the artist WF Wakeman as frequently wearing "a huge pair of green spectacles", padded shirts to hide his malnourished figure, and a hat which "resembled those which broomstick-riding witches are usually represented with". In 1849, weakened by poverty, alcoholism and malnutrition, he succumbed to cholera aged only 46. He was buried in Glasnevin Cemetery.