He intended to follow this with Give Your Heart to the Hawks, a story about mountain man Jedediah Smith in the 1820s based a novel by Winfred Blevins "It's my interpretation of Jedediah Smith, which might not be exactly historical," said Milius. "It'll be about exploration, about the need to see what's over the next ridge and what that does, what price you pay, to find out. Like Dirty Harry, Smith is a classic lone man, with a searing loneliness about him. A leader of men is always alone." It was never made; neither was Man-Eaters of Kumoan (1976) based on book by Jim Corbett about a tiger hunter in India which Milius worked on.

The A Team made a number of movies not directed by Milius. Notably, they produced the first three films from Robert Zemeckis and Bob Gale: (I Wanna Hold Your Hand, 1941 (directed by Steven Spielberg) and Used Cars. He also produced Hardcore, directed by friend Paul Schrader.

Milius began to get writing commissions: he did The Texans for Al Ruddy at Paramount, a contemporary version of Red River (1948) (never made although Sam Peckinpah was going to direct it in 1979) – Milius later said it "wasn't very good". He also wrote an original called Truck Driver (aka The Haul) which was purchased by Levy-Garner-Laven. Neither was made.

Milius says he attempted to join the Marine Corps and volunteer for Vietnam War Service in the late 1960s, but was rejected due to a "chronic" and "sometimes disabling" case of mild asthma.

Milius reflected his "ambitions stopped at B Westerns ... I thought that was a good life. I never wanted to be Hitchcock or some big mogul, I didn't want to be Louis B. Mayer. I wanted to be ... Budd Boetticher or something ... John Ford." His short films at film school included The Reversal of Richard Sun (1966), Glut (1967) and Viking Women Don't Care (1967). He wrote a documentary, The Emperor (1967), directed by classmate George Lucas, who also edited an animated short Milius directed called Marcello I'm So Bored (1967) with John Strawbridge.

He has two children by his first wife, Renee Fabri (m. 7 January 1967), and one child by his second wife, Celia Kaye (m. 26 February 1978).

Milius' name had been mentioned in a 1968 Time magazine article about the new generation of Hollywood filmmakers, which also referred to George Lucas and Martin Scorsese. This was read by Mike Medavoy, who became Milius' agent. Medavoy called Milius "a badboy mad genius in a teenager's body, but he was a good and fast Writer with original ideas."

He followed this with The Last Resort which was optioned by Michael S Laughlin in 1969. Milius says, "Neither of them were ever made, but I was able to option them. I had them rented out for like $5,000 a year."

A film based on his 1970s script, Extreme Prejudice, released in 1987, though directed by Walter Hill. There was some talk he would direct a movie for HBO, Capone, but it was not made.

George Hamilton hired him to rewrite Evel Knievel (1971), a biopic of the stunt rider. He wrote an original script The Life and Times of Judge Roy Bean about the famous judge. He sold it to First Artists for $300,000, then extremely high for a script. This was compensation for Milius not being allowed to direct. Directed by John Huston and starring Paul Newman it was a moderate hit, although Milius disliked the final result. More popular was Jeremiah Johnson.

The character of John Milner from the 1973 George Lucas film American Graffiti was inspired by Milius, who was a good friend of Lucas while they were at USC film school. Likewise, the character Walter Sobchak in the 1998 film The Big Lebowski, made by his friends the Coen Brothers, was partly based on Milius. The novella "Blind Jozef Pronek and Dead Souls" by Aleksandar Hemon features an episode with Milius, who is described as "sitting at a desk sucking on a cigar as long as a walking stick."

In 1974 David Picker announced he would produce Ranch Life and the Hunting Trail directed by Milius and written by Winfred Blevins, about Theodore Roosevelt. The film was never made. Neither was The Life and Times of Joe McCarthy, a proposed biopic about the famous anti-Communist Senator, which Milius declared interest in making.

In 1975, Milius formed his own production company, The A Team, with Buzz Feitshans, who had edited Dillinger. They had a five-year deal with Warner Bros. Milius said, "Our motto is Civitas Sine Prudentia, which really translates to Social Irresponsibility; I believe in it. It's refreshing, it's liberating. Americans are basically socially irresponsible ... Who else would have invented the atomic bomb quite the same way? The Nazis would have invented it with the Desire to conquer the world; we were the only people that could have invented it with the Desire not to conquer the world"

However the following year saw the release of Apocalypse Now, directed by Francis Ford Coppola. Coppola rewrote the script, which Milius disliked. "He wanted to ruin it, liberalize it, and turn it into Hair," said Milius in 1976. "He sees himself as a great humanitarian, an enlightened soul who will tell you such wonderful things as he does at the end of GODFATHER II-that crime doesn't pay ... Talent-wise, he's no John Ford; character-wise, he's no Steve Spielberg. Francis can't stand to have any other creative influence around ... Francis Coppola has this compelling Desire to save humanity when the man is a raving fascist, the Bay Area Mussolini."

Milius' old agent, Mike Medavoy, helped establish Orion Pictures in 1978 and one of their first movies was going to be East of Suez, written and directed by Milius. It was not made.

In the late 1980s Milius tried to get funding for adaptations of Allan Eckert's "The Frontiersman," about settling the Ohio River Valley, and "Half of the Sky," about a Rocky Mountain Explorer.

Milius enjoyed his greatest commercial success as a Director with Conan the Barbarian (1982), which made a star of Arnold Schwarzenegger.

He helped produce Uncommon Valor (1983) and acted as "spiritual adviser" for Lone Wolf McQuade (1983). He wrote and directed an episode for The New Twilight Zone (1985) and a story of his, "Viking Bikers from Hell" was used in an episode of Miami Vice (1987).

This was followed by the popular, if controversial, Red Dawn (1984).

In 1986 it was reported he was writing the script for Fatal Beauty which he hoped to direct with Cher; the film was made by Tom Holland starring Whoopi Goldberg.

Milius suffered a major financial Reversal in the late 1990s and early 2000s when his accountant embezzled funds from him – an estimated $3 million in all.

Milius is Jewish and has been married three times. His current marriage (since 1992) is to Actress Elan Oberon (who appeared in Red Dawn as the woman behind the counter at the store, his 1989 film Farewell to the King and who is seen – and heard – singing Garryowen in Rough Riders).

In 1993 he replaced Andrei Konchalovsky as Director on The Northmen for Morgan Creek Productions, about an English monk who gets captured by a band of Vikings. "This was inevitable," Milius said of his directing a Viking film. "I've been a practicing pagan for a long time. Conan the Barbarian was really a Viking movie but it was disguised." However financing fell through. He was going to direct an adaptation of Tom Clancy's novel Without Remorse with Gary Sinise and Laurence Fishburne but the project folded in 1995 two weeks before shooting was to commence due to the financial collapse of Savoy Pictures.

He also directed two films for cable: Motorcycle Gang (1994) and Rough Riders (1997). As of 2016 they are the last films Milius has directed.

After his work on Rough Riders (1997), Milius became an instrumental force in lobbying Congress to award President Theodore Roosevelt the Medal of Honor (posthumously), for acts of conspicuous gallantry while in combat on San Juan Hill. Milius made two films featuring Roosevelt: The Wind and the Lion (where he was played by Brian Keith) and the made-for-TV film Rough Riders (where Tom Berenger took the role).

That year he also wrote two biopics: Le May for Robert Zemeckis, about Curtis Le May, and Manila John, about John Basilone, which he was going to make for HBO. Warner Bros wanted him to update Dirty Harry and he wanted them to fund a version of The Iliad; there was also talk he would make The Alamo for HBO. In the early 2000s he worked on King Conan: Crown of Iron (2001–02), a sequel to Conan the Barbarian. He also developed Jornada del Muerto (Journey of Death) (2003), a biker film starring Triple H and wrote a pilot for a TV show for UPN, Delta, about a military special ops team that takes on terrorists. None of these movies were made.

Milius has long claimed to be an outsider in Hollywood. In 2001 he stated:

In 2007, Milius was the recipient of the Austin Film Festival's Distinguished Screenwriter Award. He said his favourite films were The Wind and the Lion, Big Wednesday and Conan.

In 2010 Milius was working on a new project, a film biography of Genghis Khan, and a proposed TV series called Pharaoh, set during the reign of Queen Hatshepsut, when he had a stroke. For a while he was unable to speak or move, but he ultimately recovered.

In March 2011, Milius was a story consultant for the video game Homefront, about a North Korean conquest of America.

In 2013 a documentary about his life, titled Milius, was released.

"I'd like to be Jack Hawkins in Bridge on the River Kwai," said Milius. "I call myself romantic. I believe in a lot of 19th century ideals: chivalry, honour, loyalty, romantic love."