He was a Duke of Płock, Rawa Mazowiecka, Belz, Płońsk, Zawkrze and Wizna during 1455-1461/62 (under regency until 1459) jointly with his brother, since 1459 ruler over Gostynin.
After the death of their father on 11/12 December 1455, Siemowit VI and his brother Władysław II inherited their domains; however, because at that moment they are minors, the regency was held by their mother and Paweł Giżycki, Bishop of Płock.
At the beginning of 1459 Siemowit VI attained his majority and began his personal rule according to law, however in practice his regents continue to had the real power.
The first independent move of Siemowit VI took place at the convention of Czersk, where he, together with Duke Konrad III the Red, made a peace treaty with the Teutonic Order.
The charges of murder were directly towards the Castellan of Sochaczew, Gotard of Rybna, who allegedly was offended because the princes deprived him of a country state.