Sebastian Mielitz was born in Zehdenick, and began his football career at Eintracht Oranienburg, moving later to MSV Neuruppin and Energie Cottbus.

[3] On 10 November 2007, he debuted for the second team of Werder Bremen in the second half of a match, replacing the injured first-choice goalkeeper Nico Pellatz.

[7][citation needed] Two seasons later, he made his first appearance with Werder's first team on 3 December 2009 in a Europa League home match against Nacional, filling in for the injured Tim Wiese.

[23] After starting the season on the substitute bench, he made his first appearance on 17 September 2011 coming on as a replacement for Wiese, who had been sent off in the early minutes of the match, and conceding a goal in the second–half from Philipp Wollscheid, in a 1–1 draw against 1.

[24] While Wiese served a three-match ban until late October, Mielitz played the next three matches and filled in showing very good performances, including in a win over Hertha BSC on 25 September 2011.

FSV Mainz 05 on 30 March 2013, he put out his best performance at Werder Bremen, making a lot of saves, to help the side ease their chance to avoid relegation.

[citation needed] In the 2013–14 season, Mielitz continued to remain the club's first choice goalkeeper[36] and started well keeping two clean sheets in the first two league matches against Eintracht Braunschweig[37] and Augsburg.

[39][40] However, as in the previous season, Mielitz continuously conceded goals on a regular basis, leading Kicker to say that he lacked "charisma, though he seems calm, factual, sometimes a little too pale.

FSV Mainz on 24 November 2013,[42] he was dropped as a first choice goalkeeper in favour of Raphael Wolf and was placed on the substitute bench for the rest of the season.

[46] Mielitz made his SC Freiburg debut in the first round of the DFB-Pokal, in a 2–0 win over Eintracht Trier 05, keeping a clean sheet.

[48] However, he struggled to get more playing time at the club, with newly Roman Bürki being preferred as the first-choice goalkeeper he stayed on the substitute bench throughout the season.

[49] With a lack of first team opportunities at SC Freiburg, Mielitz opted to join Greuther Fürth on 11 June 2015, signing a two–year contract.

[50] Upon joining the club, he was expected by then-manager Stefan Ruthenbeck, who was appointed one day before he signed for them, to fight for the first-choice goalkeeper role.

[51] Mielitz made his Greuther Fürth debut in the opening game of the season, keeping a clean sheet in a 1–0 win over Karlsruher SC.

[59] Ahead of the 2016–17 season, the club signed three goalkeepers, Balázs Megyeri, Sascha Burchert and Marius Funk, providing more competition for Mielitz.

[60] Mielitz made his debut for SønderjyskE in the opening game of the season, in a 0–0 draw against Randers and keeping a clean sheet in the process.

60 second into a friendly pre-season game in July 2021 against Borussia Mönchengladbach, Mielitz twisted his ankle and tore his anterior and posterior syndesmotic ligament, which kept him out for the rest of 2021.

[65] On 28 June 2022, Helsingør confirmed that, by mutual agreement, Mielitz' contract had been germinated as he wanted to return to Germany to be closer to his family.