In 1374 he inherited Zakroczym and in 1381 the lands of Czersk, Liw and Wyszogród as the result of partition of Masovia between his younger brother Siemowit IV and himself.
Originally, their main settlements were in the area of Płock, Łomża, Wizna, Czersk, Ciechanów, Płońsk, Zakroczym and Grójec.
As the oldest son Siemowit obtained the central part of Masovia, with Rawa Mazowiecka (capital) Sochaczew, Zakroczym, Gostynin, Ciechanów and Wizna.
When Casimir III the Great died in 1370, under agreements concluded with one of his lifetime friends Siemowit III, he freed Siemowit from Polish suzerainty and gave Płock, Wizna, Wyszogród and Zakroczym back to Mazovia.