In 2002 he was cast for two seasons in UK show The Tribe, a post apocalyptic series about a virus that has wiped out all adults, leaving only young people to fight amongst each other. In 2004 he was cast for one season as a lead actor on US show Power Rangers Dino Thunder, playing Conner McKnight, a soccer player and the Red Ranger. The show was filmed in New Zealand and featured an almost entirely New Zealand cast, including friend and future film producer Tom Hern.

In 2010 he started work on his second feature film, The Dark Horse, based on the life story of Genesis Potini, a Māori chess player who suffered from severe bipolar. Napier Robertson stated he became very close friends with Potini, playing hundreds of games of chess with him while writing the screenplay. Napier Robertson has been an avid chess player since a young age, which he cited as one of the things that inspired him to make The Dark Horse.

The Dark Horse premiered as the Opening Night film at the NZIFF Film Festival on 17 July 2014, and was released across New Zealand on 31 July. The film was a New Zealand box office hit, grossing $2 million and receiving unanimously rave reviews. The New Zealand Herald rated it 5 stars, calling it “a great, deeply affecting movie”, with “brave, assured and layered directing” and praising the “towering performance of Cliff Curtis”. A review on Radio New Zealand declared it to be “one of the greatest New Zealand films ever made”.

The Dark Horse was released worldwide in 2015, and was released theatrically in the United States in 2016 where it was presented by James Cameron.

Napier Robertson has stated in interviews he has been a vegetarian since the age of four due to his love for animals, and has been a member of Greenpeace for most of his life. Animal advocacy group SAFE named him ‘Hottest Vegetarian of 2014’ alongside Academy Award winner Anna Paquin as a way of raising awareness for reducing animal cruelty. Napier Robertson is married with two young children a son born in 2016 and a daughter named Sophia who live with him in Auckland.