Killen joined Oldham Athletic in July 2002 for £250,000, but failed to match his price tag because of persistent injury, and was eventually released on a free transfer.

His first senior club appearances came during a loan spell at Second Division side Wrexham in September 2000, then managed by Brian Flynn.

[8] In November 2001, he was recalled from his loan by manager Kevin Keegan as a number of Manchester City strikers were injured, and played as a substitute in a 2–1 First Division win against Rotherham United.

[10] He joined Oldham Athletic in July 2002,[11] after manager Iain Dowie spent £250,000 to acquire the young striker.

[14] He made his debut against Hearts on 28 January,[15] and then scored on his second appearance as he netted the third goal in Hibs' 3–0 defeat of Rangers at Ibrox in the Scottish Cup fourth round on 4 February.

Killen scored the opening goal in a 5–0 win over Latvian club Dinaburg in the UEFA Intertoto Cup second-round game at Easter Road on 2 July 2006.

[18] Following the departure of strikers Garry O'Connor and Derek Riordan, Killen was expected to be an important player for Hibs in the 2006–07 season.

[19] During a 2–2 Scottish Cup draw with Aberdeen in January, however, Killen suffered a ruptured Achilles tendon which ended his season.

[20] Killen's contract with Hibernian expired in the summer of 2007 and he rejected the offer of a one-year extension from the Edinburgh club.

[22] Killen made his debut on 5 August, coming off the bench in a goalless draw against Kilmarnock at Celtic Park.

[24] He made his UEFA Champions League debut on 3 October in a shock 2–1 victory over AC Milan at Celtic Park.

[32] Killen returned to England in January 2010 and joined his former Celtic manager Gordon Strachan at Middlesbrough, along with teammates Willo Flood and Barry Robson.

After being released by Middlesbrough, Killen signed a year and half contract with Chinese Super League side Shenzhen Ruby in July 2010, then managed by Siniša Gogić.

[37][38] Killen scored his first goal for the "Youth Army" on his debut against Liaoning Whowin at the Tiexi New District Sports Center on 28 July 2010.

[39] He finished his first half-year at Shenzhen Stadium with eight goals from thirteen matches to help the club to avoid relegation by a two-point margin.

[44][45] Killen featured in New Zealand's success at the 2008 OFC Nations Cup, though he played just one game – the 2–0 win over Fiji at Churchill Park.