Milius began his career in screenwriting in the late 1960s, writing short stories before moving to larger productions, writing the screenplay for the movies Evel Knievel and Dirty Harry (not credited for Harry, though he did write Clint Eastwood's speech about the .44 Magnum and if the prisoner felt "lucky") and two other Harry movies: Magnum Force and Sudden Impact (writing the line "Go ahead, make my day!").
His other movie credits include Apocalypse Now, 1941, Conan The Barbarian, Red Dawn, Clear and Present Danger, and Geronimo: An American Legend. He also wrote Robert Shaw's famous speech in Jaws about the sinking of the USS Indianapolis and the forthcoming shark attacks.
His TV credits were the TV movie Rough Riders a documentary, Between The Lines: The Story of Surfers and the Vietnam War, and created the series Rome that ran on cable and the BBC from 2005-07.
Milius married Renee Fabri in 1967 until their 1978 divorce, they have two children; Ethan and Marco, later marrying actress Celia Kaye (Big Wednesday) in 1978, later divorcing. In 1992 he married actress Elan Oberon.