Born and raised in Thornton Heath, London, the second of four children born to Marie (née Martyn) and Liam Rooney Sr., Irish immigrants from Sligo and Westmeath, respectively, Rooney started running for Croydon Harriers just before starting his secondary school, The John Fisher School.

Rooney was selected to represent Great Britain at the 2005 European Juniors, held in Lithuania in July of that year.

Despite completing his leg in 44.8 seconds, Martyn and teammates Timothy Benjamin, Robert Tobin and Malachi Davis came in fourth, with a sub three-minute time.

[citation needed] Despite impressing everyone throughout the heats, he came fifth in the final with a time of 45.51 seconds and set a new British Junior Record, previous held by Roger Black for 20 years.

[citation needed] It was announced on 13 September 2016 that the Russian athlete Denis Alexee, had failed a retest of his sample from the 2008 Olympics.

This means that Rooney will be upgraded to a bronze medal from the 4 × 400 m along with Michael Bingham, Robert Tobin and Andrew Steele.

Rooney had a disappointing 2011, knocked out in the semi-finals of the 400 metres at the World Championships in Daegu and finishing eighth as part of the British team in the 4 × 400 m relay.

In the 4 × 400 metres relay final Martyn received the baton in sixth place and ran a blistering 44.1 second anchor leg, but was just unable to overtake Deon Lendore of Trinidad and so finished fourth.