Killeen first held political office in 1985 when he was elected to Clare County Council and served on that body until 1997.

In 2004, he was appointed as Minister of State at the Department of Enterprise, Trade and Employment with special responsibility for Labour Affairs.

In January 2007, it emerged that Killeen's office had sent letters to the Minister for Justice, Equality and Law Reform advocating for the early release of a convicted child rapist.

[4] On 23 March 2010, Taoiseach Brian Cowen appointed Killeen as Minister of Defence replacing Willie O'Dea who had resigned for committing perjury in front of the High Court.

[5] On 6 January 2011, Killeen announced his decision not to contest the 2011 general election, citing medical advice, having been diagnosed with bowel cancer in 2008.