Guildford first represented New Zealand at the U19 World Cup in 2007. He then represented New Zealand at the JWC in 2008. In his final season at International age grade level the following year, Guildford again played for New Zealand, in that year's JWC hosted in Japan. Tragically, his father Robert died suddenly shortly after the conclusion of the Final against England. Guildford was first selected for the All Blacks for the end of year tour in 2009 and was the youngest player in the squad. He made his All Blacks debut against Wales on 7 November 2009 at Millennium Stadium, Cardiff. In 2010 he was selected for the Steinlager Series. Guildford was selected for the All Blacks tri-nations squad the following year and was later selected in the 30-man All Blacks 2011 Rugby World Cup squad, though he did not play until New Zealand's final pool match of the tournament, where he contributed four tries to the All Blacks' 79-15 victory over Canada. Guildford was then recalled into the All Blacks for the three test Ireland series where he played two more tests on the right wing.
Super Rugby
After a strong season with Hawke's Bay in the Air New Zealand Cup, Guildford was chosen for the Hurricanes2008 season. Guildford managed to play six matches in the 2008 Super 14 season, starting all of these matches except his first match against the Stormers. Guildford also managed to score three tries in his first season. In 2009 he played in 10 matches, most of them as a starter, scoring 4 tries. In 2009 he was secured by the Crusaders for the 2010 season. Guildford signed for the NSW Waratahs for the 2016 super rugby season.
Hawke's Bay
Guildford was one of the youngest players to play in the Air New Zealand Cup. He made his debut for Hawke's Bay in 2007 when he was only 18. Being the youngest player in the squad didn't stop him though, as he managed to play in twelve of the provinces matches in a year that they made the semi-finals. Guildford also managed to run in five tries for the season, two coming in the 30–26 win over Waikato. Guildford went on to be one of the leading try scorers in 2009 with 13 tries, most of them through his trade mark kick and chase.
Clermont Auvergne
In January 2014, it was announced that he signed a contract for two years with French Top 14 side Clermont Auvergne. Initially, he was supposed to join them at the conclusion of the 2014 Super Rugby season, but in May 2014, he was given an early release from his contract by the.
Sevens
Guildford was included in the New Zealand sevens squad that competed in the 2010 Commonwealth Games in New Delhi. They won the Gold Medal against Australia.
Controversy surrounding alcoholism
On 11 November 2011, Guildford was cited in Rarotonga, where he was accused of drunken naked assaults during an alcohol-fuelled series of incidents. He has since apologised for his actions and admitted to alcoholism.