Selected out of the Taradale Rugby Club, Guildford made his debut for
Hawke's Bay in 2007 aged 18, making him one of the youngest players to play in the
Air New Zealand Cup. In his debut season for Hawke's Bay, he played 14 games and scored eight tries, two of them being scored in a 30-26 win over
Waikato. After a strong season with
Hawke's Bay in the previous year, Guildford was selected for the
Hurricanes 2008 season at age 18, making him New Zealand's youngest Super Rugby player at the time. Guildford later debuted for the Hurricanes at the age of 19 years and 70 days, making him the youngest on debut for the franchise also, beating the previous record held by
Christian Cullen by almost a year. Due to the four-match ban given to him by
New Zealand Rugby, Guildford missed the Crusaders' three pre-season matches and their first competition match against the
Blues in the
2012 Super Rugby season. With fellow wings
Ben Smith,
Julian Savea,
Charles Piutau,
Cory Jane, and
Frank Halai being preferred by the All Blacks, Guildford sought an overseas contract. In January 2014, it was announced that Guildford signed a contract for two years (with the option of a third) with French
Top 14 side
Clermont Auvergne. Guildford did not play any Super Rugby matches in 2014, instead playing club rugby in Christchurch and Napier, He was meant to join Clermont at the conclusion of the
2014 Super Rugby season, but in May 2014, he was released early from his contract by the due to his poor fitness. Guildford was set to miss up to four weeks of rugby for Clermont after an assault on him left him with a badly bruised jaw. In early May 2015, Guildford announced that he had ended his contract with Clermont Auvergne, citing the distance from New Zealand and the different style of northern hemisphere rugby as reasons for his return to New Zealand. However, it was later revealed that it was actually Clermont who terminated Guildford's contract after he tested positive for
cocaine. Guildford played in four pre-season games for Hawke's Bay in 2015, Shortly after his return to New Zealand,
Waratahs assistant coach
Daryl Gibson, who Guildford played under during their time at the Crusaders, contacted Guildford with an offer of joining the Waratahs for the
2016 Super Rugby season. Despite the speculation, the Waratahs announced that they had signed Guildford on a ten-month contract in September and was due to move to
Sydney at the end of October. Guildford broke his toe during the Waratahs' game against the
Rebels, ruling him out of the Waratahs' next game against the
Brumbies. Guildford was dropped mid-season from the Waratahs squad, forcing him to play in the
Shute Shield for
West Harbour RFC, but later earned a recall into the team. In June, Guildford left the Waratahs for "wellbeing reasons".
Tasman announced in May that they had signed Guildford for the
2016 Mitre 10 Cup season to replace injured wings
Viliami Lolohea and
James Lowe. However, in August and before the start of the season, he and Tasman agreed to "part ways". Days after his contract with Tasman was cancelled, Guildford signed with
Heartland Championship team
Wairarapa Bush, Following the Heartland Championship, Guildford played in sevens tournaments in
Singapore and
Kuala Lumpur. Through their mutual ties to
Hawke's Bay, coach
Roger Randle influenced Guildford to move to Waikato at the end of 2016. In December, Randle named Guildford in Waikato's 12-man sevens squad to compete at the Northern Regional Seven's tournament in
Cambridge. Guildford had an impressive performance for the Waikato sevens team at the national competition, where they finished runners-up behind Counties Manakau. Following the sevens season over the summer, Guildford played for Hamilton Old Boys Rugby Club in the Waikato premier competition and was selected by Waikato for the
2017 Mitre 10 Cup. Playing at fullback, he suffered a hamstring injury during Waikato's 23-10 defeat to
Manawatu in
Hamilton and was replaced by Tyler Campbell. The injury ruled him out of selection for their next game against
Wellington. Despite a strong Mitre 10 Cup performance for Waikato, he was not signed by any Super Rugby teams for the
2018 Super Rugby season, a result that Guildford attributed to "[coaches and
New Zealand Rugby] still looking at the old Zac Guildford". Following the 2017 Mitre 10 Cup season, Guildford was named in the Waikato sevens team a second time to compete in the Northern Regional Seven's tournament. Guildford was named in the wider training squad for
Wairarapa Bush in May 2021. == International career ==