From 1960 to 1967 he studied theology at the seminary of the Evangelical Lutheran Church of Latvia in Riga.

On 8 June 1969 he was ordained a priest in Holy Trinity Lutheran Church, Riga, to which he had been a vicar since 1968.

Mesters was elected archbishop of Riga on 15 April 1986 during the extraordinary synod of the Latvian Evangelical Lutheran Church.

On 24 August he was consecrated by Olof Sundby Archbishop emeritus of Uppsala in St. Gertrude New Church, Riga.

In the turbulent eighties in the Baltic states, when young people from the three countries in particular called for independence, Mesters supported the Soviet government.