Little is known about Mercurius, son of Projectus, other than that he was a Roman. And Mercurius is not known to Western culture under his given name. We know him as Pope John II, who took the throne, January 2, 533.
Mercurius was a priest at St. Clement's Basilica on the slope of Mons Coelus. This was a rich, fashionable residential area of the city. Remnants of the basilica still retain memorials reading "Johannes surnamed Mercurius". There was also found a fragment of a ciborium with the words "Presbyter Mercurius".
In the past generation or so before Pope Boniface II, simony had become a very difficult problem in the Catholic Church. It was not only the selling of positions or indulgences, but, during the two month vacancy after the death of Boniface, it included the selling of sacred vessels. The last decree which the Roman Senate ever issued, passed under Boniface II, was against simony in papal elections. This decree was confirmed by the Gothic King Athalaric, at his court in Ravenna. In addition to confirming this, he put in an addendum ordering it engraved in marble and placed in the atrium of St. Peter's Church, in 533. Apparently he was pretty tired of dealing with the scandals of the Roman Church, for he added another comment. If a disputed election was brought to Ravenna by the Roman clergy, 3000 soldi would be charged and the proceeds distributed to the poor.
We assume that there was no disputed election when Pope John II ascended the throne. John maintained good terms with the king and Athelaric, with the pope, so much so that all complaints against the Roman clergy were sent to John's tribunal rather than Athelaric's court.
In 451, at the Council of Chalcedon, Nestorianism was condemned. This heresy separated the divine and human natures of Christ so much so that it denied that the Virgin Mary could be called the Mother of God. A year after John became pope, Emperor Justinian the Great asked John to condemn the Acoerneti. These were monks in Constantinople who had adopted the heresy of Nestorianism. John rightly excommunicated them on March 24, 534. In thanksgiving, Justinian sent John a profession of his orthodox faith. This was a great accomplishment for the orthodoxy considering the strength of monophytism in Byzantian lands.
John had to face another problem, closer to home. The bishop of Riez, Provence, named Contumeliosus, was accused of a scandalous adultery. John had various options opened to him. He chose wisely. He ordered the bishops of Gaul to confine the, now, ex-bishop to a monastery. And the laity and clergy of that diocese was to obey the bishop of Arles until a new bishop could arrive.
The Council of Carthage (535) showed that North Africa was still wrangling the problem of Arian lapsi. 217 bishops came to the council to as a question: On repentance, can bishops who have lapsed retain their position or be admitted only to lay communion? After almost 100 years the problem still was not settled. John died before answering and it was up to his successor, Agapetus, to answer the question.
John died May 8, 535, having served the Church almost two and a half years. He was buried in St. Peter's Basilica.