A product of Reading's Academy, McCarthy spent time out on loan before signing professional terms with the club in 2008. He made his debut in 2009 whilst on loan to Aldershot Town and spent the following season at Yeovil Town before making his Reading debut in 2011. He later spent time on loan at Leeds United and Ipswich Town. McCarthy left for QPR in 2014, moving on to Crystal Palace the following year, before joining Southampton in August 2016.
At international level McCarthy has represented England and made his debut for the under-21 team in 2010, appearing three times in total. In 2018, he made his debut for the senior team.