He established the Kęsgailos family in Samogitia, where their power rivaled that of the Grand Duke of Lithuania. Mykolas Kęsgaila was the deputy of Ukmergė (1409–1412), Elder of Samogitia (1412–1432, 1440–1441, and 1443–1450), and castellan of Vilnius (1443–1448). He was a father of Jonas Kęsgaila and Mykolas Kęsgaila the Younger.
Mykolas presided over Christianization of Samogitia in 1413, subdued a peasant rebellion in 1418, and foiled a plot by nobles against Vytautas in 1419.
Rumbaudas and Jaunius were executed, while Mykolas managed to restore his power and domains. Mykolas witnessed the Union of Grodno of 1432 between Sigismund and Jagiełło, King of Poland.
According to the Bychowiec Chronicle, after the murder of Sigismund in 1440, Mykolas was among supporters of Casimir IV Jagiellon for the throne of Grand Duke. Foreign such support, he was reinstated as Elder of Samogitia, but the Samogitians did not want to recognize Casimir"s authority.
As a compromise, Casimir granted a privilege affirming semi-autonomic status of the region and granting new freedoms to the nobles.
Mykolas was able to return to the office.