[1] His coaching career started when Siemieniec was a student at a University of Physical Education in Katowice.
On 4 April 2023, following Maciej Stolarczyk's dismissal, he took on the role of interim manager of Jagiellonia's senior team until the end of the season.
[4] After Jagiellonia avoided relegation at the end of the 2022–23 season, Siemieniec's appointment was made permanent.
[citation needed] In January 2024, it was announced that Siemieniec prolonged his contract for additional two years.
[5][6] In the 2023–24 campaign, Siemieniec led Jagiellonia to an unlikely title win, clinching their first-ever top-flight championship with a 3–0 home victory over Warta Poznań on the last matchday of the season.