After the death of his father in 1267, Siemowit, together with his full-brothers, inherited their share of his lands under the regency of their mother until 1275, when they jointly ruled.
Around 1287 as a result of the customary divisionary treaty between his brothers, he received the district of Dobrzyń The government of the frontiers of his Duchy weren't easy for Siemowit, and in 1293 he was captured by the Lithuanians during an invasion.
During his absence, the rule was exercised by his brother Władysław I the Elbow-high, who after his return handed back the power to Siemowit.
His refusal to continue to pay the tithes, caused that Siemowit and his family where excommunicated; the curse was removed only after the Duke's death in 1316.
They had five children: Because his sons are minor at the time of his death, the regency was exercised jointly by his widow and his brother Władysław I.