In his 40s, he was a monk, the abbot of Santa Maria in Trastavere, but was used by the popes for various missions and tasks. He translated many Greek works into Latin, and his style of writing shows up in several documents that were supposedly written by popes. (It is possible that he took their dictation and put his own style into what they wanted to say rather than taking it down verbatim.)
Holy Roman Emperor Louis II sent him to Constantinople to help negotiate a marriage between Louis' daughter Ermengard and Constantine, eldest son of Eastern Emperor Basil I. This was in 869, and the Fourth Council of Constantinople was taking place when they arrived. Anastasius attended the final session and defended the papal demands to have more jurisdiction over Bulgaria and the East. The marriage negotiations failed, or maybe they didn't; historians argue about that.
As the papal legates returned from that Council, the document with all the decisions was stolen from them. A copy of the declarations in Greek was in the possession of Anastasius, however, who was able to deliver his copy to Rome and translate it into Latin. The original Greek version is lost. The Council had deposed patriarch Photios, but Anastasius kept in touch with him.
After Pope Nicholas died and Adrian II became pope, Anastasius was named official papal librarian, hence the epithet Bibliothecarius. Anastasius was implicated in a plot that killed Adrian's wife and daughter, but Adrian's successor, Pope John VIII (872 - 882) confirmed him as librarian and encouraged him to write. The illustration shows an early vellum page with part of Anastasius' history of the Byzantine Church.
(A contemporary, Archbishop Hincmar of Reims, writing a history of the time, claims that Anastasius was the same Anastasius who was briefly an anti-pope at the time of Pope Benedict III (855 - 858). That Anastasius was driven from Rome in 848, excommunicated in 850 by the Roman synod, nd deposed by another synod in 853. It seems unlikely that he would have been welcomed into the papacy a couple decades later.)
In 879, a new librarian appears in papal records, Zacharias of Anagni, so by that time Anastasius had probably died or simply retired.
So what happened to poor Ermengard, daughter of the emperor, who was looking for a husband and did not find one in Constantinople? We'll see what happened to her tomorrow.