Some historians believe that Siemowit rendered a tribute of vassalage to Charles IV, Holy Roman Emperor in 1346.

In addition, Casimir III offered Siemowit a small territory of Lesser Poland bounded by the river Pilica and Radomka.

In 1363, Siemowit was invited to Kraków for the marriage of Elizabeth of Pomerania, granddaughter of the king of Poland, with the Emperor Charles IV.

In September 1364 he participated in a conference in Kraków, which offered five crowned kings, dukes and princes extravagant festivities and tournaments.

Having a reunified Masovia, it regained its independence, there Siemowit III promulgates a customary law in 1377.

After working his whole life on reuniting Masovia and making it an independent state, Siemowit III died on 16 June 1381, and was buried at Plock Cathedral.