McKidd was born on 9 August 1973, in Elgin, Moray, Scotland, the son of Kathleen, a secretary, and Neil McKidd, a plumber. He grew up in a council estate near Inverness. At 17, McKidd worked at the Macallan distillery in Speyside. He later went to work with the Lumsden family of coppersmiths. He attended Seafield Primary School and Elgin Academy. and was a member of the local amateur dramatic group, Moray Youth Theatre. Planning to study engineering, he initially attended the University of Edinburgh, then decided to enroll at Edinburgh’s Queen Margaret College to study drama. He joined Edinburgh University’s student theatre company, Bedlam Theatre, where he was a member of the improvisational comedy troupe The Improverts.

After playing Tommy Mackenzie in Trainspotting, McKidd was cast as Father Deegan in the 1996 Christmas episode of Father Ted. Subsequent roles include Malky Johnson in Small Faces. In 2004, he played James Hepburn, 4th Earl of Bothwell, the third husband of Mary, Queen of Scots, in the BBC mini-series Gunpowder, Treason & Plot.

In 1998, McKidd was in the four part BBC2 series Looking After Jo Jo, playing the role of Basil. He was also one of the main stars of the joint HBO/BBC series Rome, where his portrayal of the soldier-politician Lucius Vorenus received critical acclaim.

In 1999 he was part of an ensemble cast in Mike Leigh’s Topsy Turvy, depicting the development of Gilbert and Sullivan’s The Mikado. The actors researched their historical characters and used this knowledge during extensive rehearsals to help develop dialogue for scenes blocked out by Leigh. McKidd, like the rest of the cast, did his own singing in the Gilbert and Sullivan operettas portrayed in the film.

McKidd married Jane Parker in 1999. They have two children, a daughter and a son. In August 2015, McKidd and his family became U.S. citizens. The couple said in July 2016 that they were separating and have been divorced since December 2017.

In 2001 he played Elliot in the British film Understanding Jane, directed by Caleb Lindsay. He appeared in Ridley Scott’s Kingdom of Heaven, and in the 2002 film adaptation of Nicholas Nickleby. In Neil Marshall’s horror film Dog Soldiers, he appeared as Pte Lawrence Cooper. In the 2005 BBC drama, The Virgin Queen, he played Thomas Howard, 4th Duke of Norfolk.

Additional work includes his role in the Silence of the Lambs prequel Hannibal Rising (2007). In late 2007, McKidd began his role as the lead character in the American fantasy television series Journeyman on NBC. Despite starting out with a strong audience, the show lost about half of its viewership throughout its run and suffered from the fractious situation in the United States due to the writer’s strike at the time. Thirteen episodes were produced.

McKidd has done voice work and voiced the character of Jezz Torrent, flame haired lead singer of the fictional Scottish hard rock band Love Fist in the video game Grand Theft Auto: Vice City. He is credited with playing a voice-over for the character John “Soap” MacTavish in Call of Duty: Modern Warfare 2 and Call of Duty: Modern Warfare 3. In 2012, he voiced the characters of Lord MacGuffin and his son Young MacGuffin in the Disney/Pixar film Brave. Having grown up in Elgin, McKidd used a variation of the Doric dialect for Young MacGuffin, and one of the running gags of his lines is that not even Lord MacGuffin is entirely sure what Young MacGuffin is saying.

McKidd instigated The Speyside Sessions, a Scottish folk music album recorded in 2011/12 on Hogmanay in his home town of Elgin. Many of the contributors to the album are old school friends of McKidd’s. The album was released on 15 June 2012 in aid of Save the Children.

In April 2013, McKidd was Grand Marshall of the 15th annual Tartan Day Parade in New York City. At the time, he voiced his support for Scottish independence and expressed regret that he would be unable to vote in the then upcoming Scottish independence referendum the following year because he no longer lives in Scotland. Of Scottish independence, McKidd stated, “A lot of people think we’re all gonna fall flat on our face if we do this. We’re too feisty a people to let things turn bad if we went and tried it. Why not? Why not try at least?”

McKidd announced in March 2018 that he and chef Arielle Goldrath were married in a Jewish ceremony. On 13 May 2018, they welcomed a son. McKidd announced that their daughter Nava was born July 27, 2019.