Mathieu attended St George's High School in the Bahamas, graduating in 2001 as their "Most Outstanding Athlete".

He earned All-American honours by reaching the NCAA indoor relay final, then achieved the same feat in the outdoor season.

The 2006 indoor season was his last at Texas Tech: he set a 200 m personal best of 21.34 seconds and again reached the NCAA relay final.

[2] He reached the senior international level at the 2007 World Championships and became a fixture in the Bahamian national team from then onwards.

He was selected for the national team at the 2009 World Championships in Athletics, but disqualifications meant he was out of the first round of both events.

He performed better at the 2010 IAAF World Indoor Championships, reaching the semi-finals, but the team failed to finish the relay.

He came close to medal at the 2011 Pan American Games, where he was fourth behind Brazil's Bruno de Barros.

[9] He also won gold at the 2012 London Olympics with the Bahamas 4 × 400 m team beating medal favorites USA with a national record.