After giving a year of engineering school a shot, McKidd decided to focus on his passion and changed his major at Edinburgh's Queen Margaret College to drama. While in his final year of studies, he landed a starring role in "The Silver Darlings" at Citizen's Theatre in Glasgow. Soon he was spotted by Gillies MacKinnon and cast in the 1960s housing scheme set "Small Faces", playing the hard leader of the Tongs, a gang from the rough side of Glasgow in this moving coming of age story. The actor next co-starred in "Trainspotting", Danny Boyle's popular and acclaimed film adaptation of the Irvine Welsh cult novel exploring a group of friends, some joined by tradition, some by heroin. McKidd played Tommy, one of the five friends who initially steers clear of drugs, only to be sucked in by desperation and curiosity when his girlfriend breaks up with him. His character served as the pure-hearted and tragic victim of the film, succumbing to heroin addiction and stricken with AIDS. McKidd could next be seen in John Duigan's "The Leading Man" (1996, released in the USA in 1998) before reuniting with Gillies MacKinnon and "Trainspotting" co-star Jonny Lee Miller for a supporting role in the World War I psychodrama "Regeneration" (1997, released in the USA in 1998). He followed with a starring role as a hard-nosed thug opposite veteran criminal Patrick Stewart in the brutal but energetic crime thriller "Dad Savage.”
McKidd proved his range with a subtle and endearing portrayal of a lovelorn, British middle-class gay man as the star of Rose Troche's "Bedrooms and Hallways", before retreading Irvine Welsh ground with a role as hapless Johnny, the sucker star of "The Soft Touch", one of three darkly comedic Welsh story adaptations that made up the compilation film "The Acid House" (both 1998, both released in the USA in 1999). "Understanding Jane", a 1998 winner of the Audience Award for the best British film at the London Film Festival, starred McKidd as a sex-obsessed slacker who meets his match through the personal ads and he offered a near wordless cameo as a fellow hitchhiker to Kate Winslet her two daughters in Gillies MacKinnon's "Hideous Kinky" (1998, released in the USA in 1999). In 1999, he was featured alongside Dexter Fletcher and Jim Broadbent in “Topsy-Turvy,” a biopic of composers Gilbert and Sullivan helmed by Mike Leigh.
McKidd made the infrequent television appearance with a performance as the handsome and charming Count Vronsky in Masterpiece Theatre’s presentation of “Anna Karenina” (PBS, 2000). Returning to features, he was seen in the adaptation of Charles Dickens’ “Nicholas Nickelby” (2002), then played post-expressionist artist George Grosz in “Max” (2002), a speculative drama about a fictional relationship between Adolf Hitler and an influential Jewish art dealer (John Cusack) who fails to support the artist’s work, forcing him to channel his creative energy into hating Jews and Germany’s uncertain future. In the straight-to-video “Dog Soldiers” (2002), he did battle with his superior officer (Sean Pertwee) and a gang of werewolves in a remote Scottish forest while on a training exercise with a group of soldiers. He then starred in “Afterlife” (2003) as an ambitious journalist forced to care for his sister, who has Down syndrome, after learning his mother (Lindsay Duncan) is dying from cancer. McKidd next starred as a troubled young man dealing with alcoholism and life as a violent gang member in Richard Jobson’s semi-autobiographical “Sixteen Years of Alcohol” (2003).
In “Once Last Chance” (2004), McKidd played a barman trapped in a small Scottish Highland’s town with his two best friends (Jamie Sives and Iain Robertson). He next appeared in “De-Lovely” (2004) as the male lover of famed songwriter Cole Porter who creates a stir with the composer’s wife (Ashley Judd) after revealing a compromising photo. A small role in Ridley Scott’s flailing Crusade actioner “Kingdom of Heaven” (2005) prepared the actor for his part on HBO’s acclaimed historical series, “Rome” (2005- ). Kidd played the honorable, but unforgiving Lucius Vorenus, a severe professional Roman soldier who stays above the fray of rape and pillage by saving his honor and dignity for his wife on his return home from the war in Gaul. The grueling 14-month shoot—of which his was involved every day—was wearying and the grandeur of the set—and ostensibly the project—overwhelmed him. Earlier in his career, he may well have gotten paranoid and ran away. But an older, more mature actor hunkered down and convinced himself not to “muck it up.” Meanwhile, McKidd appeared as the Duke of Norfolk in “The Virgin Queen” (PBS, 2005), Masterpiece Theatre’s sprawling biopic on the life of Queen Elizabeth I.