Mefferts first club was the TV Hoffnungsthal. Here he played until 2005. Afterwards he moved to the Bundesliga club Bayer 04 Leverkusen. There he went through all the junior teams. On 17 November 2012, he first played for the second team of Leverkusen.
Less than a week later he celebrated his pro debut when he came on as a substitute in the Europa League game against Metalist Kharkiv in the 65th minute for Stefan Reinartz. Since the 2013/14 season Meffert belongs firmly to the senior squad.
Meffert played some games for the various youth national teams of the DFB. Overall, he played ten times for various youth teams, but not score.