Slater has a well tabloid-publicized record of alcohol and drug abuse, and a series of high profile romantic relationships. He dated Christy Turlington in 1995. He splits his time between New York City, Los Angeles, California, and London, England. He has two children by his ex-wife Ryan Haddon: Jaden Zach Haddon-Slater (born April 6, 1999), and Eliana Sophia (born August 15, 2001). Slater divorced Haddon in November 2006.

Slater has had several altercations with the police and other authorities. On August 11, 1997, he was arrested by police in Los Angeles and charged with three counts of assault with a deadly weapon and one count of battery. Slater, drunk and high on heroin, punched his then-girlfriend, fashion editor Michelle Jonas, in the face at a party and bit a man who attempted to protect her. Slater did not go quietly, but hid in a stairwell and fought with officers, reportedly shouting, "The Germans are coming and they will kill us!" He then attacked a police officer.