Rösler ya shiga FDP da kungiyar matasa na siyasa, 'Yan Matasa na' yanci, a 1992. Ya kasance sakataren FDP a jihar Lower Saxony daga 2000 zuwa 2004 kuma ya yi aiki a matsayin shugaban kungiyar majalisar FDP a majalisar jihar Lower Saxony daga 2003. Daga 2001 zuwa 2006, Rösler ya kasance memba a majalisar yanki na Hanover (gundumar), inda kuma ya kasance mataimakin shugaban kungiyar majalisar. A watan Mayu 2005, an zabe shi mai lura da kwamitin zartarwa na FDP na tarayya. Ya sami kashi 95% na kuri'u, sakamakon mafi kyau na wannan taron jam'iyyar. A taron jam'iyyar na jihar a watan Maris na 2006, an zabi Rösler a matsayin shugaban FDP na Lower Saxony tare da kashi 96.4% na kuri'un; ya yi nasara Walter Hirche, wanda ya sauka bayan shekaru goma sha biyu a kan karagar mulki. A watan Afrilu 2008, an tabbatar da Rösler a matsayin shugaban jam'iyyar FDP na Lower Saxony. Tambaya: Lokacin da ya karbi kashi 95% na kuri'un, ya yi nasara?
Shekarar 1992