Mercury excelled as both a running back and kick returner. The majority of his playing days were spent with the Miami Dolphins. From 1969 to 1971, he backed up Jim Kiick at halfback and served as the Dolphins' primary kickoff return man. In his rookie year of 1969, Mercury averaged 26.4 yards per kickoff return, leading the AFL in kickoff returns with 43 and in kickoff return yardage with 1136. Both totals would have also led the NFL. His 105-yard return was the longest in the AFL that season, and he was also one of the AFL's leading punt returners that year. In 1970, he missed some time on the field due to a leg injury, but his 6.8 yard per carry average on 60 runs was the highest in the league among players with at least 50 runs.