Mitchell-Blake was recruited by Louisiana State University and began to compete for them collegiately with the LSU Tigers team. In his first year in 2013 he was a 200 m semi-finalist at the NCAA Outdoor Championships and took fourth in the 4 × 100 metres relay there. In the relay he had top three finishes in the Southeastern Conference in both the 4 × 100 metres relay and 4 × 400 metres relay. He represented Great Britain at the 2013 European Athletics Junior Championships and was the 200 m champion, having set a best of 20.62 seconds. He also helped his country to fifth in the relay.