[3][11][12] Siddig's first television role was a Palestinian man in a British six-part miniseries called The Big Battalions (filmed in 1989 but released in 1992), and shortly afterward, he won the role of Prince Feisal in A Dangerous Man: Lawrence After Arabia, a 1990 telefilm sequel to Lawrence of Arabia starring Ralph Fiennes.

[3][12] Siddig's role in A Dangerous Man: Lawrence After Arabia brought him to the attention of Rick Berman, executive producer of Star Trek: Deep Space Nine (1993).

[4] Siddig told Bidoun magazine that demand for Islamic and Arabic character roles was increasing in both film and television after the September 11 attacks and that people began to approach him with projects within six months of the event.

[1][12] In 2003, Siddig played the role of an Algerian secret agent on the trail of Islamists in the controversial episode "Nest of Angels" of the British television show Spooks (known as MI-5 in the US).

[15][16] Siddig appeared in a cameo role as Khalid Sheikh Mohammed the film The Hamburg Cell that premiered the following year.

[23][12] In 2009, he co-starred with Patricia Clarkson in the award-winning film Cairo Time as Tareq Khalifa, an Egyptian who battles his attraction to his friend's wife.

[29][30] From 2013 to 2015, Siddig played Aslan Al-Rahim ("the Turk") in the BBC historical fantasy drama series Da Vinci's Demons.

[33] Siddig also provided the voice of Wolf, a character in the historical fantasy drama Tumanbay that aired on BBC Radio 4 between December 2015 and February 2016.

In 2017, Siddig joined the cast of the televisions series Gotham as League of Shadows leader Ra’s al Ghul[34][35] and also played the part of Aristotle Onassis in the drama miniseries The Kennedys: After Camelot.

[37][38] That same year, he appeared as Issouf Al Moctar in the second season of the Fox (UK and Ireland) thriller series Deep State, and voiced Avi Singh in Star Wars: The Bad Batch.