Anastasios Kephalas, later Nectarios, was born on 1 October 1846 in Selymbria, to a poor family. His parents, Dimos and Maria Kephalas, were pious Christians but not wealthy.

He served as a bishop in Cairo for one year. Nectarios was very popular with the people, which gave rise to jealousy among his colleagues. They were able to persuade his superior that Nectarios had ambitions to displace the Patriarch. Nectarios was suspended from his post without explanation. He then returned to Greece in 1891, and spent several years as a preacher (1891–1894). He was then director of the Rizarios Ecclesiastical School for the education of priests in Athens for fifteen years. He developed many courses of study, and wrote numerous books, while preaching widely throughout Athens.

In December 1908, at the age of 62, Nectarios resigned from his post as school director and withdrew to the Holy Trinity Convent on Aegina, where he lived out the rest of his life as a monk. He wrote, published, preached, and heard confessions. He also tended the gardens, carried stones, and helped with the construction of the monastery buildings that were built with his own funds.