Born to Joseph Blake and Audrey Mitchell-Blake in Newham, London, his family relocated to Jamaica when he was age thirteen. Having already competed extensively as a child in the UK, his talent for sprinting was recognised by Jamaica College, who took him on, and he competed at the Inter-Secondary Schools Boys and Girls Championships. After a 200 m win at the 2011 Jamaican junior championships in a personal best of 21.54 seconds, he was selected to compete for Great Britain at the 2011 World Youth Championships in Athletics and finished fifth in his semi-final. 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. In his second year at Louisiana he failed to improve after an injury-affected indoor season, having a best of 20.69 seconds. He was again a semi-finalist at the 2014 NCAA Outdoor Championships, though he claimed his first top three finish there with LSU in the 4 × 100 metres relay. Upon his return to competition in 2016, he set new bests. In the 60 metres he ran 6.65 seconds and in the indoor 200 m he completed the distance in 20.57 seconds. At the Southeastern Conference indoor championships he won the 200 m after another best of 20.51 and also placed seventh in the 60 m. He entered the longer event at the 2016 NCAA Division I Indoor Track and Field Championships and finished as runner-up. In the outdoor season, he ran 10.09 seconds for the 100 metres in April, putting him at the top of the seasonal British rankings. He marked a breakthrough at the SEC Outdoor Championships, coming within one hundredth of the British record for the 200 m with his winning time of 19.95 seconds. He completed a regional triple by taking the 100 m individual and relay events, too. Mitchell-Blake anchored the men's 4 x 100m relay team to a gold medal at the 2017 World Championships. He ran the same leg at the 2019 World Championships, with the team finishing in silver medal position with a new European record of 37.36 seconds.
