In August, 1942, the German occupational authorities banned Mstyslav from Kiev General-Governorate. As Mstyslav disobeyed the order, he was arrested in Rivne. On Gestapo accusations he spent half a year imprisoned in Chernihiv and Pryluky. He was freed in Spring 1943 but was ordered not to leave Kiev and banned from conducting the religious services.

In 1949 he also became the metropolitan of the [[Ukrainian Orthodox Church of the USA]]. At the 1949 Council in New York he succeeded in bringing about unification with the eparchy of Bishop Ioan Teodorovych, who became [[Metropolitan]] of the UOC of USA. Bishop Mstyslav became his deputy and the head of the consistory. In the US, Bishop Mstyslav began extensive church activity with the Ukrainian Orthodox Center, a publishing house, library and seminary being built in South Bound Brook, New Jersey. In 1969 his authority was extended over the Ukrainian Autocephalous Orthodox Churches of Europe and Australia. During his meetings with Ecumenical Patriarch in 1963 and 1971 he brought up the issue of the canonical recognition of the Ukrainian Diaspora churches (UAOC was banned in the USSR, and hence in Soviet Ukraine at that time).

In 1990 he returned to Ukraine where at his age of 92 he was elected the first Patriarch of Kiev and all Ukraine of the UAOC following its controversial and short-lived union with the recently proclaimed Ukrainian Orthodox Church - Kiev Patriarchy. He was enthroned as Patriach Mstyslav on November 6, 1990.