As commander in the field in 1257 against Michael II, despot of Epirus, he showed little military ability. He was captured and kept for two years in prison, from which he was released by Michael Palaeologus. Meanwhile, Michael Palaeologus was proclaimed emperor of Nicaea, afterwards expelling the Latins from Constantinople, and became emperor of the whole East; and from this moment Acropolites becomes known in the history of the eastern empire as one of the greatest diplomatists. After having discharged the function of ambassador at the court of Constantine, king of the Bulgarians, he retired for some years from public affairs, and made the instruction of youth his sole occupation.
But he was soon employed in a very important negotiation. Michael, afraid of a new Latin invasion, proposed to Pope Clement IV to reunite the Greek and the Latin Churches; and negotiations ensued which were carried on during the reign of five popes, Clement IV, Gregory X, John XXI, Nicolaus III, and Martin IV, and the happy result of which was almost entirely owing to the skill of Acropolites.
As early as 1273 Acropolites was sent to pope Gregory X, and in 1274, at the Second Council of Lyon, he confirmed by an oath in the emperor's name that that confession of faith which had been previously sent to Constantinople by the pope had been adopted by the Greeks. The reunion of the two churches was afterwards broken off, but not through the fault of Acropolita. In 1282, Acropolites was once more sent to Bulgaria, and shortly after his return he died, in the month of December of the same year.
Acropolites is considered a trustworthy authority as far as the statement of facts is concerned, and he is easy to understand, although he exhibits special carelessness in the construction of his sentences. His style is archaising but lucid. He was also the author of several shorter works, amongst them being a funeral oration on John Vatatzes, an epitaph on his wife Irene Lascarina and a panegyric of Theodore II Lascaris of Nicaea. While a prisoner at Epirus he wrote two treatises on the procession of the Holy Spirit.