Born in Sudan, Siddig was raised in Britain and attended the London Academy of Music and Dramatic Arts (LAMDA) where he studied acting and theater. Immediately after leaving LAMDA, Sid did a season of theater in Manchester, in addition to performing in various shows in pubs and small theaters.
Later, Siddig worked as a director in a small theater in London. "I was in a situation where I pretty much had a theater company at my disposal and a theater at my disposal and could do anything I liked... as long as I did it for nothing." During this time of financial struggle Sid was offered the part of "Emir Feisal" in Great Performances: A Dangerous Man: Lawrence After Arabia (1992), the sequel to Lawrence of Arabia (1962), in which he co-starred with Ralph Fiennes. Siddig's performance brought him to the attention of Rick Berman who was creating the new series Star Trek: Deep Space Nine (1993).
During his time on DS9, Siddig and co-star Nana Visitor had a son (Django) and were married in 1997. They divorced in 2001, although they remain friends.
After a successful seven-year run on television, Siddig began working in feature films, quickly accumulating roles in movies such as Vertical Limit (2000), Reign of Fire (2002), Kingdom of Heaven (2005), and the critically acclaimed Syriana (2005). Early 2005 saw Siddig return to the stage opposite Kim Cattrall in the West End production of "Whose Life Is It Anyway?"