Mieciel was born on December 22, 1975 in Gdynia, Poland. He played for several clubs in his native country (Wisla Tczew, Lechia Gdansk, Hutnik Warsaw, Legia and Lodski) before playing abroad for Borussia Möchengladbach in 1996. He returned to Poland and Legia, for the 2002 season and, in the summer of that year, Mieciel came to Greece to play for Iraklis Thessaloniki. He recorded 16 goals in 64 appearances for Iraklis before transferring to PAOK in 2005. He left the club in 2007.
Mieciel has since played in Germany with VfL Bochum and in his native Poland before he retired in 2012.
Mieciel was capped 5 times for the national team of Poland, scoring one goal (against Cyprus).