After the death of Pope Leo IV in 855 this Anastasius was elected as Antipope by the imperial party, but the rightfully elected Pope Benedict III gained the supremacy, and acted kindly towards the usurper.
During the pontificate of Pope Adrian II, Anastasius became involved in serious difficulties when, in 868, his brother Eleutherius forcibly carried off the daughter of the Pope and soon afterwards killed her and her mother.