Smith grew up in the town of Bullaburra in the Blue Mountains outside Sydney. He graduated from Blue Mountains Grammar School in 2002. As a teenager, he trained with the Sydney Kings NBL basketball team.

Smith resumed playing rugby competitively in 2008. Within weeks of starting he played for the All-American Team and following his graduation, had started to play with the Denver Barbarians. Soon after joining the Barbarians, he was named 2008 Rugby All-American.

In international rugby, Smith was a regular with the US national team, for which he qualified on residency grounds. He made his test debut on 8 November 2008 against Uruguay. Smith helped his country qualify for the 2011 Rugby World Cup during which he started all 4 group stage matches for the United States. After the 2011 Rugby World Cup, Smith took a hiatus from rugby.

The New York Jets signed Smith on 3 April 2012. He was waived on 31 August 2012. A day later, he was signed to the Jets’ practice squad. On 27 October 2012, he was promoted to the active roster from the practice squad. He caught his first and only NFL pass on 23 December 2012. He was released on 26 August 2013.

On 9 December 2013 it was announced that Hayden had re-signed for Saracens on a long term deal. In January 2014, Smith played his first professional rugby match for Saracens since returning from the NFL. Smith started for Saracens in the 2014–15 Anglo-Welsh Cup Final helping them claim a victory at the death, 23–20 over Exeter Chiefs.

Smith resumed playing for the United States in March 2014 in the qualifying matches for the 2015 Rugby World Cup. He also played for the US in the June 2014 Pacific Nations Cup.