Isidore moved up the career ladder rather quickly, primarily due to the classical education he had received. Researchers speak of various educational centers of late Byzantine time, among which the leading were Mystras and Constantinople. The unique handwriting of Isidore was formed probably in Mystras, the capital of the Byzantine Peloponnese (Morea) and the largest center for Greek manuscripts rewriting; Constantinople provided to Isidore a chance to be taught by such outstanding teachers as Manuel and John Chrysoloras or John Chortas-men, although it is impossible to say for sure who taught Isidore. One way or another, at the end of his staying at Constantinople Isidore proved himself as an encomiast due to his education and entered the immediate circle of Emperor Manuel II of Palaeologus, functioning as the chief scribe — a copyist of Emperor's writings, and also, as some scholars note, being a co-worker of this "philosopher on the throne". Then Isidore retired and spent more than two decades at the Peloponnese, where he took the monastic vows in the Contostephanos monastery in honor of the Holy Archangel Michael and All the Bodiless Powers of Heaven (near Monemvasia); he served Cyril, the Metropolitan of Monemvasia. Bearing his obedience, Isidore manifested himself as a hymnographer, panegyrist and scientist; on the Metropolitan's instructions Isidore was engaged in the compilation of historical-canonical texts designed to settle canonical questions of the dioceses belonged to the metropolia. Apparently, at the same time Isidore met George Gemistus Plethon, an outstanding philosopher, who influenced his worldview in a significant way.

The future Metropolitan of Kiev was a typical representative of the intellectual environment of late Byzantine society, practicing various forms of intellectual communication, common in his time. Thus, Isidore corresponded with different persons: he wrote to his fellow students in Constantinople as well as to the Byzantine emperors Manuel II and his son John VIII Palaeologus. The analysis of these writings and also the character of Isidore's education, lead to the conclusion that Isidore was not just an ordinary Byzantine intellectual, but a humanist with a specific attitude to science, philosophy and religion, who belonged to elite groups, the so-called "theaters" or "salons", in which the intellectuals used to discuss a wide range of scientific issues.

The turning point in the life of a future Kiev Metropolitan was the appointment of Isidore to the post of Kathegoumenos of the Emperor's Monastery of St. Demetrius in Constantinople and his further participation as an ambassador in the work of Basel Council, which was devoted to the discussion of a project of Ecumenical Council intended to overcome the schism between the Western and Eastern Churches. Having analyzed Isidore's speech at the Basel Council, the author concludes that it was here that Isidore first claimed the idea of Union which determined his future and turned to be in some sense his lifeline: shortly after Isidore returned from Basel, he was elected the Metropolitan of Kiev so that he could ensure the representation of the Russian Church at the Union Council and later implement the Union.

The "Russian" period of Isidore's life is also reflected in historical sources. The analysis of these sometimes very laconic references, as well as a set of documents on the history of the Ferrara-Florentine Council of 1438–1439 indicates that Isidore was almost silent, with the exception of a few insignificant remarks. However, approximately from the spring of 1439 and till the Union was signed, Isidore together with another Metropolitan, Bessarion of Nicaea, undertook a series of practical actions aimed at inducing the Orthodox clergy to join the Union, thanks to which the unification was achieved, albeit at the expense of the purity of the Orthodox faith. In this case, Isidore consistently pursued the policy of unification in accordance with a key strategy of Byzantine Emperor John VIII Palaeologus, which, apparently, was in harmony with his own convictions. Motives of the Kiev Metropolitan's actions should be sought, first of all, in a pragmatic political consideration and intention to achieve unity between the Churches with the main purpose — to save Byzantium, dying in the struggle against the Turks, with military support from the West. In this regard Isidore behaved as a typical Hellenic patriot who was ready to do anything to save his homeland.

In the areas of his legacy, as well as on the territory of the Byzantine Empire in the 1440's, Isidore promoted the Florentine Union, using his own method of persuasion, in accordance with which he attracted the Orthodox Christians appealing exclusively to the Byzantine patristic heritage. In the territories controlled by Isidore as the Metropolitan of Kiev, the news of the unification of the Churches was perceived with suspicion and skepticism, and somewhere it was encountering great resistance. A severe conflict caused by the announced unification broke out in Moscow between Grand Prince Vasily Vasilyevich and Metropolitan Cardinal Isidore, with the result that the latter was imprisoned and deposed by the Russian bishops' council. It was for the first time that Russian people faced such situation that is why, probably, they let Isidore to escape. Isidore's character and his activity in Russia were remembered by Russian people for a long time, and as soon as his name was mentioned in some work, it was accompanied from now by an unflattering characteristic reflecting rather negative attitude towards Isidore. The Church of Constantinople represented in Russia by Isidore was also compromised, which undoubtedly played a role in establishing autocephaly of the Russian Church in 1448 under St. Iona (Jonas) of Moscow.

Study of the last period of Isidore's life is associated with a number of problems, the unambiguous resolution of which sometimes has seemed unattainable. First of all, they are: lack of published sources on the Roman Curia life in the 1440–60's, and in general insufficient historiography information concerning the convocation of the Council of Mantua