Efter examen från Royal Academy of Dramatic Art, år 2003 dök Armstrong upp i Quartermaine's Villkor vid Kungliga Teatern i Northampton som Derek Meadle och i mars 2004 spelade han Henrik i huden på våra tänder vid Young Vic teatern i London Borough of Lambeth. År 2004 medverkade han i sex avsnitt av den fjärde säsongen av den brittiska tv-komedi-drama serien, lärare på Channel 4, som Anthony Millington.I januari 2005 medverkade han som Richard i Rutherford & son vid Royal Exchange i Manchester. Senare under 2005 uppträdde han åter på Channel 4 i brottet dramaserien The Ghost Squad som Pete Maitland. I december 2006 fick han huvudrollen i två delar brottslighet drama Losing Gemma på ITV. 
Hans första stora TV-roll kom i oktober 2006 när han spelade Robin av Locksley i BBC: s moderna anpassning av Robin Hood historien. Under inspelningen av den andra serien, som sändes under 2007 så bröt Armstrong ett Mellanfots ben i foten under en iscensatt kamp scen. Armstrong var en gäst panelmedlem på BBC komedi panel tävling Never Mind the Buzzcocks den 28 februari 2007.

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Jonas Armstrong (born 1 January 1981 in Dublin) is an Irish born British actor, raised in Lytham St Annes, Lancashire best known for his appearances on television in the United Kingdom, where he has played the title role in the BBC One drama series Robin Hood.

After graduating from the Royal Academy of Dramatic Art, in 2003 Armstrong appeared in Quartermaine's Terms at the Royal Theatre in Northampton as Derek Meadle. And in March 2004 he played Henry in The Skin of Our Teeth at the Young Vic theatre in the London Borough of Lambeth. In 2004 he appeared in six episodes of the fourth series of the British television comedy-drama, Teachers on Channel 4, as Anthony Millington. In January 2005 he appeared as Richard in Rutherford & Son at the Royal Exchange in Manchester. Later in 2005 he appeared again on Channel 4 in the crime drama series The Ghost Squad as Pete Maitland. In December 2006 he starred in the two-part crime drama Losing Gemma on ITV.

His first major television role came in October 2006 when he played Robin of Locksley, in the BBC's modern adaptation of the Robin Hood story. During filming of the second series, which aired in 2007, Armstrong broke a metatarsal bone in his foot during a staged fight scene. Armstrong was a guest panelist on the BBC comedy panel game show Never Mind the Buzzcocks on 28 February 2007.

In August 2008, the BBC confirmed that Armstrong would be leaving Robin Hood at the end of the third series, which aired on June 27 2009, with him saying that he was "looking for new challenges" and the BBC saying "he'll be desperately missed". In 2008 he appeared in the horror film Book of Blood which is based on a short story byClive Barker.

He appeared in Episode 3 of the third series of BBC serial The Street broadcast on 27 July 2009, playing the role of TA soldier Private Nick Calshaw who returns from Afghanistan with a facial disfigurement and a prosthetic hand after being injured by a suicide bomber whom he was unable to shoot.