[9] On 27 March 1930, Pope Pius XI appointed him Apostolic Vicar of Dar es Salaam, making him a bishop and assigning him the titular see of Vinda.
[9] Within a decade he increased the number of mission stations from eight to seventeen and improved living conditions for their staff, with particular attention to their health.
During World War II, he worked out an arrangement for exchanging staff with German Benedictines so the British authorities would not deport them.
[10] Pope Pius XII named him the first archbishop of Dar es Salaam when he erected that archdiocese on 25 March 1953.
[6] As archbishop he aimed at establishing a "Eurocentric culture", staging large events where "well-behaved students, disciplined cadets and churchgoers dressed in European clothes accompanied his magnificent entry into the cathedral of Dar es Salaam with a long train on festive days".
Following the achievement of independence by former European colonies and the creation of Tanzania, Maranta offered his resignation to allow for the appointment of an indigenous archbishop.