After working at the New Zealand National Film Unit as a director, Neill was cast as the lead in 1977 New Zealand film Sleeping Dogs. Following this he appeared in Australian romance My Brilliant Career (1979), opposite Judy Davis.
In the late 1970s, his mentor was James Mason. In 1981 he won his first big international role, as Damien Thorn, son of the devil, in Omen III: The Final Conflict. Later, Neill was also one of the leading candidates to succeed Roger Moore in the role of James Bond, but lost out to Timothy Dalton. His Bond screen-test can be found on the special features of the The Living Daylights (1987) DVD.
Among his many Australian roles is playing Michael Chamberlain in Evil Angels (1988) (released as A Cry In The Dark outside of Australia and New Zealand ) about the case of Azaria Chamberlain.
Neill has played heroes and occasionally villains in a succession of film and television dramas and comedies. In the UK he won early fame, and was Golden-Globe nominated, after portraying real-life spy Sidney Reilly in mini-series Reilly, Ace of Spies (1983). His leading and co-starring roles in films include thriller Dead Calm (1989), two-part historical epic La Révolution française (1989)(as Marquis de Lafayette), The Hunt for Red October (1990), Jurassic Park (1993), Sirens (1994), Rudyard Kipling's The Jungle Book (1994), John Carpenter's In the Mouth of Madness (1995), Event Horizon (1997), Bicentennial Man (1999), and comedy The Dish (2000).
Neill has also occasionally acted in New Zealand films, notably The Piano (1993), which marked the first time a woman director (Jane Campion) had won the top prize at the Cannes Film Festival. His other New Zealand features include Gaylene Preston's genre-crossing 'romance' Perfect Strangers, and a 2009 adaptation of science fiction tale Under the Mountain. Neill himself returned to directing in 1995 with documentary Cinema of Unease: A Personal Journey by Sam Neill (1995) which he wrote and directed with Judy Rymer. Made as part of a BFI series marking the centenary of cinema, the film saw Neill providing his own take on New Zealand film history.
In 1993, Neill co-starred with Anne Archer in Question of Faith, an independent drama based on a true story about one woman's fight to beat cancer and have a baby.
In 2002 he hosted and narrated a documentary series for the BBC entitled Space (Hyperspace in the United States).
Neill also portrayed the legendary wizard in Merlin (1998), a miniseries based on the legends of King Arthur. He reprised his role as Merlin in the sequel, Merlin's Apprentice (2006), in which Merlin learns he fathered a son with the Lady of the Lake.
Neill starred in the historical drama The Tudors, playing Cardinal Thomas Wolsey. "I have to say I really enjoyed making The Tudors", Neill said,“It was six months with a character that I found immensely intriguing, with a cast that I liked very much and with a story I found very compelling. It has elements that are hard to beat: revenge and betrayal, lust and treason, all the things that make for good stories."
He also acted in short-lived Fox TV series Alcatraz as Emerson Hauser. By May 2012, he was working on fantasy adventure movie Mariah Mundi and the Midas Box throughout the South West of England, playing the role of Otto Luger. The movie is scheduled for release in 2013.