Cullen was born in Paraparaumu and grew up in Paekakariki, a small town north of Wellington. He is the youngest of three children. He has an elder twin brother named Shane and a sister named Anita. Cullen is of Irish descent; he also has Samoan and German ancestry.
His rugby talent emerged in his high school years and he was selected in the New Zealand secondary schools team in 1993 and 1994. He played senior rugby for Manawatu in 1995 and scored 70 points through 12 tries, two conversions and two penalty goals. He was selected to the New Zealand sevens squad for tournaments in Fiji and Hong Kong.
Cullen's provincial career started in 1996 season. He played in the first-ever Super 12 game, contested by the Hurricanes and the Blues. That year he scored seven tries in nine matches.
At the 1996 World Sevens competition in Hong Kong he scored 18 tries, including seven in one match. Later that year he was selected as an All Black. He scored seven tries in his first two Test matches: a hat-trick on debut against Samoa and four tries against Scotland. He sustained a knee cartilage injury on the All Blacks' tour to South Africa.
In 1997 Cullen returned from injury and scored 11 tries in 10 games for the Hurricanes, and 12 tries in 12 Test matches for the All Blacks.
The All Blacks had a poor year in 1998 with five consecutive losses, but Cullen scored four tries in seven matches. He represented New Zealand in rugby sevens at the Kuala Lumpur Commonwealth Games, playing seven games, scoring nine tries and kicking 23 conversions. New Zealand won the tournament and the gold medal.
In 1999 Cullen was a member of New Zealand's unsuccessful Rugby World Cup squad. He played six matches in the Cup tournament, starting five at centre. In 2000 he scored 10 tries in 11 games for the Hurricanes and seven tries in four Tri Nations Tests, including three consecutive pairs.
In 2001 Cullen sustained a severe knee injury that required surgery and an intensive recovery programme. He made himself unavailable for the All Blacks' 2001 end-of-year tour, but was announced ‘dropped’ by coach John Mitchell at a NZRU press conference. Cullen and Mitchell's personal differences went unresolved over time. Cullen was selected to and dropped from the All Blacks squad throughout 2002. He scored four tries in the five Test matches that year.
In the 2003 Super season Cullen scored eight tries in 12 games for the Hurricanes, taking his Super Rugby career tally to 56, a record at the time. He was briefly and controversially selected for the Māori rugby team, despite having, according to his father, about 1/64th Māori ancestry. He was left out of New Zealand's 2003 World Cup squad. He played out the NPC season for Wellington and received a standing ovation after the Championship final, despite Wellington's losing to Auckland.
At the end of 2003 Cullen moved to Ireland, where he played for Munster. His time with Munster was blighted by injuries, particularly to his shoulder, limiting his appearances. His last match for Munster was a 15–7 Celtic League win over the Newport Gwent Dragons at Musgrave Park on 28 April 2007. Two weeks later, on 12 May, he announced his retirement from rugby and stated his intention of returning to New Zealand to start a business.
Cullen is the All Blacks' second-highest try-scorer in the Tri Nations Series with 16, second only to Richie McCaw. He was the first player to score a try in every Tri Nations test in one season. He is New Zealand's second most-capped test fullback. At the time of his retirement he was the All Blacks' leading try-scorer with 46; that record has since been overtaken by Doug Howlett. He scored over 150 tries while playing in New Zealand. He is the third-highest try-scorer in Super Rugby with 56 tries, behind Joe Roff and Howlett.
His biography, Christian Cullen: Life on the Run (by John Matheson) was released in October 2003. The book received attention particularly for Cullen's criticism of John Mitchell.