Her career started in 1994 when she performed her first leading role as Juliet Hulme in Peter Jackson's Heavenly Creatures. Since then she has acted in several films, such as Sense and Sensibility, Jude, A Kid in King Arthur's Court, Titanic, Hamlet, Hideous Kinky, Holy Smoke, Iris, and The Life of David Gale, earning her three Oscar nominations and two Golden Globess.
On November 22, 1998 she married director Jim Threapleton with whom she has a daughter named Mia. After her divorce in 2001, she began a relationship with director Sam Mendes. Kate married Sam on May 24, 2003 in the West Indies and their son Joe was born on December 22, 2003.