While in high school, Dagg represented Hawke’s Bay at under-16 and under-19 level. Dagg attended the Advanced Course at the International Rugby Academy (IRANZ) in April 2006. His course facilitator was current Wallabies coach, Dave Rennie, and former Otago, Otago Highlanders and New Zealand player Jeff Wilson. That year, he became the first secondary school player to be selected for Hawke’s Bay since Danny Lee. The 2006 New Zealand Rugby Almanack named him one of its five promising players for 2006.

Dagg made his All Blacks debut against Ireland in New Plymouth on 12 June 2010. He was again capped against Wales the following week but was injured at half time.

Dagg’s second test try came in dramatic circumstances against South Africa on 21 August 2010 at FNB Stadium near Soweto, Johannesburg. With the scores level at 22-all, in the last minute of play, Dagg backed up a break by Ma’a Nonu to score the game-winning try for a 29–22 result.

Dagg had a major injury-plagued year in 2015, only managing 5 games for the Crusaders and 3 tests for the All Blacks causing Dagg to narrowly miss out on being picked for New Zealand’s 2015 Rugby World Cup squad. Dagg subsequently returned to Hawke’s Bay to play for the Magpies and scored a hat-trick against Otago, but was injured playing his 50th game. Dagg’s latest injury required shoulder surgery, ruling him out for the rest of the Magpies’ Mitre 10 cup-winning year.

Dagg finished 2016 as the highest try-scorer of the year for the All Blacks, beating the team’s new first-choice fly-half and World Rugby Player of The Year- Beauden Barrett and Ben Smith by one try. Dagg scored 10 in total from the 12 games he played in 2016.

Despite missing a chunk of the Crusaders’ Super Rugby winning campaign due to knee surgery in 2017, Dagg was retained as the first-choice right winger for the All Blacks’ series against the British and Irish Lions and Pasifica Challenge against Samoa. Dagg scored a try in the 78–0 victory against Samoa and set up debutant Vaea Fifita for his first test try. Dagg was a standout performer against the Lions in the first test, which was a 30–15 victory and in the second test he was moved to fullback which was a historic 24–21 loss to the Lions. Dagg started the third test on the wing again as he surpassed Jonah Lomu and Sir John Kirwan as the second-to-most capped All Blacks winger, second only to former teammate Joe Rokocoko.

Dagg has been married to wife Daisy since 2015. They have 2 children, a son Arlo who was born in April 2017 and a daughter, Tilly born in May 2018. In 2015, he appeared in a Men in Black themed safety ad for Air New Zealand with singer Stan Walker and actor Rip Torn.