[1] On 31 May 2003, aged 16, he made his II liga debut for Stomil Olsztyn in a game against Świt Nowy Dwór Mazowiecki, coming on as a substitute for Marcin Kubsik in 79th minute.

[5][6] He made his Ekstraklasa debut on 15 May 2004 in a match against Dyskobolia Grodzisk Wielkopolski, when he came on as a substitution for Dariusz Romuzga 86th.

[12] On 11 August 2005 he made his debut in European competition playing in a UEFA Cup second qualifying round match against Grasshopper.

[16] He made his debut for Zagłębie on 10 March in a league game against Piast Gliwice and a month later in a 3–0 victory over Kmita Zabierzów he scored his first goal for a new club.

[19] In the autumn round of the following season he scored eight goals in 20 matches including a hat-trick in an 8–0 victory over ŁKS Łomża in the Polish Cup.

[20] On 23 July 2009 he scored his first goal in a European competition in a 4–0 victory over Juveness/Dogana in the second qualifying round of the 2009–10 UEFA Europa League.

[28][29] However, the player didn't complete his transfer at that time as he was during the negotiations process with the Saudi Arabian team.

With Al-Nassr, he played in all the six group stage matches, scoring just 2 goals and failed to qualify for the Round of 16.

[33] Mierzejewski made his debut and scored his first goal for Sydney FC in the 3–0 2017 FFA Cup win over Bankstown Berries, dispatching a penalty in 90+1.

The Pole scored his first league goal from the penalty spot as well, in a 2–0 win against Perth Glory at Allianz Stadium.

In the 2017 FFA Cup Final, Mierzejewski won the Mark Viduka Medal for Man of the Match, as the Sky Blues beat Adelaide United 2–1 AET.

[34] On 29 April 2017, Mierzejewski has taken out the 2017/18 Johnny Warren medal for best player in the A-League finished 11 points clear of Dimitri Petratos and Leroy George.

[39] On 28 April 2022, Mierzejewski signed with fellow Chinese Super League club Henan Songshan Longmen.