In 1384, he escaped captivity and joined his brother Vytautas, who allied himself with the Teutonic Knights.

After the death of Vytautas, he supported his cousin Švitrigaila in his fight against Poland, but later was convinced by nobles to join a conspiracy against him.

He signed the Union of Grodno with Jogaila and ceded some territories in Volhynia and Podolia to Poland.

After strengthening his positions in Lithuania, he tried to loosen his ties with Poland and negotiated between 1438 and 1440 with Albert of Hungary (who was also the German king) for an anti-Polish alliance, but was killed by supporters of Švitrigaila (possibly led by Alexander Czartoryski) at the Trakai Peninsula Castle on 20 March 1440.

Sigismund had one son, named Michael Žygimantaitis, who died shortly before 10 February 1452.