On 31st January 2009, it was reeported that Norwich had singed Celtic's New Zealand international striker Chris Killen on loan until the end of the 2008/9 season.

Killen grew up in the Wellington suburb of Island Bay. As a youth he played for Miramar Rangers, whose coach arranged a trial with Manchester City. The trial was successful, and the 17 year old Killen joined City's youth academy. His first senior club appearances came during a loan spell at Wrexham in September 2000. He scored his first league goal on 9 September 2000 against Oldham, and in total scored three goals in nine appearances for the Welsh club. The following season he was loaned to Port Vale, scoring six goals in nine league games. In November 2001 he was recalled from loan as a number of Manchester City strikers were injured, and was a substitute in a 2–1 win against Rotherham United. he made two further substitute appearances that season, and finished as top goalscorer for the reserves.

He joined Oldham Athletic in July 2002[5] for £250,000. He remained with Oldham for 3 and a half years, scoring 24 goals in 78 league appearances. Failing to match his price tag and seemingly high wage through continuous injury, Oldham no longer required his services and released him on a free transfer.

In January 2006, Killen joined Hibernian on a free transfer. He scored on his debut as he netted the third goal in Hibs' 3–0 defeat of Rangers at Ibrox in the Scottish Cup 4th round in February 2006. His first home goal was the opener in a 7–0 win against Livingston four days later. Despite arriving in January, he had scored five goals for Hibs by the end of the season

Killen's contract with Hibernian expired in mid-2007. He rejected the offer of a one-year extension from the Edinburgh club and was expected to sign for Cardiff City before Celtic's intervention. Killen signed for Celtic on a three year contract in May 2007. Killen made his debut on 5 August 2007, coming off the bench in a goalless draw against Kilmarnock at Celtic Park. His first goal for the club came on 7 October at Fir Park; with Celtic trailing 1–0 to Gretna, Killen equalised in the 89th minute before Scott McDonald scored the winner for Celtic in stoppage time. He made his UEFA Champions League debut for Celtic at the Estádio do Sport Lisboa e Benfica against S.L. Benfica on 24 October. However he could not extablish himself as a first team regular and was linked with loan moves to Hearts and Cardiff before joining Norwch.

Killen made his international debut for New Zealand during the 2000 OFC Nations Cup against Tahiti. Killen was included in the New Zealand U-23 squad for their first appearance at the Beijing Olympic Games as one of 3 over age players

Killen made his City debut as a substitute in the 3-3 draw at Wolves on 3rd February 2009. He made just 4 appearances before returning to Scotland

On 15th January 2010, he signed for former Celtic manager Gordon Strachan's Middlesbrough on a free transfer along with fellow free transfer Willo Flood and Celtic midfielder Barry Robson for a £1million.

Killen was included in New Zealand's squad for the 2010 World Cup in South Africa.

He left Boro when his contract expired in June 2010.and signed a year and half contract with Chinese Super League side Shenzhen Ruby on July 24th 2010. Killen scored his first goal for Shenzhen on his debut against Liaoning Whowin in a Chinese Super League match on July 28th 2010. He scored eight goals from thirteen matches to help Shenzhen to remain in the Chinese Super League for the 2011 season.

He switched clubs to Chongqing F.C. in February 2012 but was released in December 2013.