Amini’s first screenplay was for the 1994 TV movie The Dying of the Light, directed by Peter Kosminsky. The TV movie told the story of Sean Devereux, an aid worker who was murdered in Somalia in 1993 for criticising arms sales. It was nominated “Best Single Drama” at the British Academy Television Awards. Amini also wrote an adapted screenplay of the 1895 novel Jude the Obscure by Thomas Hardy. Producer Andrew Eaton commissioned the screenplay in 1995, and it was filmed by Michael Winterbottom as Jude, released in 1996. Amini also wrote a screenplay for another TV movie, Deep Secrets, which aired in 1996.

Amini was one of the screenwriters, along with Michael Schiffer, who wrote a screenplay adaptation of A.E.W. Mason’s novel The Four Feathers, which became the film The Four Feathers (2002), directed by Shekhar Kapur and starring Wes Bentley and Heath Ledger. In 2008, he adapted the Elmore Leonard novel Killshot into the screenplay for the film Killshot (2008), directed by John Madden and starring Thomas Jane, Diane Lane, Mickey Rourke, Joseph Gordon-Levitt, and Rosario Dawson.

In 2010, Amini wrote an original screenplay for a film entitled Shanghai directed by Mikael Håfström and starring Chow Yun Fat, Gong Li, Ken Watanabe and John Cusack.

Amini also wrote the screenplay for the neo-noir film Drive (2011), directed by Nicolas Winding Refn. His screenplay for the film is a loose adaptation of the novel of the same name by James Sallis. During interviews, Refn noted that he and Amini cut out a lot of the content from Sallis’ book, resulting in the almost bare bones structure for the film, and the few lines of dialogue for the film’s star, Ryan Gosling. For his screenplay of Drive, Amini also won a “Best Adapted Screenplay” award from the Austin Film Critics Association in 2011.

Amini adapted John le Carré’s novel Our Kind of Traitor into a feature film, directed by Susanna White. The film, was released in 2016, starring Ewan McGregor, Naomie Harris, and Stellan Skarsgård. He then adapted The Snowman, from the novel by Jo Nesbø. It was a box office disappointment and received negative reviews, with critics calling it “clichéd and uninvolving.”

In 2018, Amini began writing for television, with the crime series McMafia and The Alienist. On 27 September 2019, it was announced that Amini will be writing the upcoming Obi-Wan Kenobi Star Wars series for Disney+.