Although Dave Armstrong is best known as a professional writer for theatre and television, he began his career as a trumpet player. After gaining a Bachelor of Music degree from Victoria University in Wellington, Dave studied in Basel, Switzerland and Los Angeles, USA.

He returned to New Zealand and performed with a number of musical groups including as a casual player with the New Zealand Symphony Orchestra and the Auckland Philharmonia, and as a member of the New Zealand Brass Quintet and Wellington Sinfonia. Two of his script adaptations, Le Bourgeois Gentilhomme and L'Histoire du Soldat have been performed by the New Zealand Chamber Orchestra in theatrical productions that Dave also directed.

Dave Armstrong is most well known for his stage plays. Westie is an adaptation of Charles Dickens A Christmas Carol, which had successful seasons at Downstage and Auckland Theatre Company in 2004. In 2005 Wellington's Circa Theatre commissioned The Tutor, which won best new New Zealand Play at the 2005 Chapman Tripp Theatre Awards. Next came King and Country; a musical play about the First World War, which has been performed at festivals throughout New Zealand, including the 2006 New Zealand International Festival of the Arts in Wellington. King and Country has been performed throughout the country and played on Radio New Zealand National. The radio adaptation was highly commended in the 2007 Media Peace Awards.

Dave Armstrong's hit Niu Sila, co-written with Laureate Oscar Kightley, played a sold-out season with the Auckland Theatre Company at AK05 (Auckland Festival) and won a Chapman Tripp Award for Best New Play. Niu Sila was performed at the 2007 Pasifika Styles Festival in Cambridge England.

In 2006, Dave and Oscar were chosen by Denis & Verna Adam to receive a donation as part of the Arts Foundation Award for Patronage. This award acknowledges an individual or couple who have contributed significantly to the arts as patrons. Denis & Verna were given $20,000 by the Foundation to distribute to artists of their choice to celebrate the occasion of the award, an amount that they doubled. Dave and Oscar received one of four donations of $10,000.

Le Sud was commissioned for Wanaka's Festival of Colour in 2009 and toured a sell-out season around the country. The play assumes that the French colonised the South Island in 1839 and are at odds with their "North Zealand" island neighbours.

Dave Armstrong has also written for television. His screen credits include Seven Periods with Mr Gormsby, which he co-wrote and co-created, and he wrote The Semisis, the Pacific Island family from Skitz. Dave was script editor for first series of Bro'town, and wrote for four series of the PR political satire Spin Doctors, for which he won an Academy of Film and Television Award for best comedy script alongside with Roger Hall and James Griffin. Other TV credits include Project Q, a children's science show, and Jandals Away.

Dave has also written a book called True Colours; an account of the 1996 New Zealand General Election. A chapter from his novel The Speechwriter was selected for the 2007 Six Pack of writing published for New Zealand Book Month.

In 2011, The Motor Camp was commissioned by Circa Theatre in Wellington. Dave wrote this play based on a short story by Danny Mulheron. The Motor Camp is a glorious celebration of annual Kiwi camping rituals.