His early life is largely unknown. Anysius was the student and co-worker of his predecessor, [[Ascholius of Thessalonica|Ascholius]]. He was a friend of St. [[Ambrose]] who congratulated him, the [[clergy]], and people of Thessalonica on his succession to the [[see]] of [[Metropolis of Thessalonica|Thessalonica]] in 383/4. <ref> Anysius was well known in that time having been mentioned by St. [[Basil the Great]] as a "beacon of light for the church", a "charismatic person".</ref> He was highly regarded by his peers.
When Eastern Illyria came under the administration of the emperor in Constantinople, Archbishop Damasus of Rome asserted his rights to preserve previous [[jurisdiction]] over Illyria by appointing Anysius his [[Vicar]] Apostolic in Illyricum. These privileges were later confirmed by Bishop [[Innocent I of Rome|Innocent]] when he succeeded to the Roman [[see]] in 401. Bp. Anysius and the bishops of Illyria condemned Bonosus, Bishop of Sardica as a [[heretic]] after his case was referred to them by the Council of Capua.