[2] He has an elder sister named Laura Jayne, who was one of the dancers featured in the music video for Eric Prydz's 2004 song "Call on Me".

After failing to participate on the first two occasions,[5] his teacher arranged for him to play the tenth juror in an adaptation of Twelve Angry Men.

Although he took part, he refused to attend a drama festival for which his teacher had also signed him up, as he saw himself as a football player and believed acting would damage his social life.

After leaving school, Smith studied Drama and Creative Writing at the University of East Anglia, graduating in 2005.

His role in the latter earned him an agent and his first professional jobs, Fresh Kills and On the Shore of the Wide World, which led him to seek an agreement with his university so that he could graduate without attending lectures in his final year.

[9] Smith's first television role was as Jim Taylor in the BBC adaptations of the Sally Lockhart quartet books The Ruby in the Smoke and The Shadow in the North.

After The History Boys, he acted in the teen play Burn/Chatroom/Citizenship and with Christian Slater in Swimming with Sharks,[13] the latter being his West End début.

[7][9] In 2007, Smith appeared as Henry in the Polly Stenham play That Face at the Royal Court Theatre Upstairs in Chelsea.

Smith discussed his character's relationship with his mother in a May 2008 interview with the Evening Standard, saying, "The thing I find tricky to get my head round is why doesn't he just leave?

[15] Upon its transfer to the West End, Smith's performance as Henry was highlighted as one of the positive aspects of the play by critics for the Evening Standard, Daily Express, The Guardian and The Times.

[16] Smith auditioned for the role of Will McKenzie in the comedy series The Inbetweeners, with the part eventually being given to comedian and actor Simon Bird.

"[17] Smith was cast in Martin McDonagh's black comedy crime In Bruges (2008), as the younger version of Ralph Fiennes' character, but his scenes did not appear in the final cut of the film.

[24] Smith was a relatively unknown actor compared to the actors then speculated about possibly taking on the role, who included Paterson Joseph, David Morrissey, Sean Pertwee, James Nesbitt, Russell Tovey, Catherine Zeta-Jones, Chiwetel Ejiofor, Robert Carlyle, and Billie Piper.

[20][21] Some fans of the show believed that Smith was inexperienced and too young for the role, while others supported him by citing his demonstrated acting ability.

"[38] In June 2010, Smith appeared on stage with Orbital, and performed with them a version of the Doctor Who theme, at the Glastonbury Festival.

[58] Smith reunited with his The Crown co-star Claire Foy in a production of the Duncan Macmillan play Lungs at The Old Vic beginning in October 2019.

[63] Smith joined the cast of Edgar Wright's psychological horror-mystery Last Night in Soho starring opposite Thomasin McKenzie and Anya Taylor-Joy.

[67] Smith joined the Sony's Spider-Man Universe spin-off film Morbius (2022) as Lucien / Milo, a living vampire, although he was initially announced to be portraying the supervillain Loxias Crown / Hunger.

[70][71] In 2024 Smith returned to the West End to star in a modern retelling of the Henrik Ibsen play An Enemy of the People.

[74] Nick Curtis of The Evening Standard gave the mixed review writing, "The casually charismatic Smith and a fine supporting cast can't stop it falling apart in the second half.

[84] He has also referred to Oasis as "the greatest rock-and-roll band in the world";[5] a friend of Noel Gallagher, he attended his 50th birthday party in 2017.