Mullins started his career with Reading, but it was during two separate loan spells with Kidderminster Harriers that he first came to prominence, prompting Mansfield Town to sign him on his release from the Reading in 2006. He was with Mansfield for two seasons before moving to Stockport County. In 2010, he joined Rotherham United, and following his loan at Oxford United he was made Rotherham's captain as they finished runners-up in League Two. In July 2013, he joined Oxford United permanently, staying for three seasons before his transfer to Luton Town.