He entered the diplomatic service of the Holy See in 1971, and worked until 1973 in the nunciature to the Sudan, which was also the apostolic delegation for the Red Sea region.

On 17 October 1987, Pope John Paul II named him Titular Archbishop of Urbs Salvia and appointed him Apostolic Nuncio to Ghana, Togo and Benin.

He was consecrated on 28 November by Cardinal Agostino Casaroli, with Bishops Albino Mensa and Luigi Bettazzi as the principal co-consecrators.

On 12 January 1990, he was transferred to Rwanda, where he supported human rights organizations and encouraged Catholic bishops to unite as forceful advocates for ending civil war.

[2] In March 1995, John Paul II appointed him Permanent Observer of the Holy See to the United Nations in Geneva from 1997, with the same role at the World Trade Organization.

On 11 January 2006, Bertello was appointed Apostolic Nuncio to Italy and the Republic of San Marino by Pope Benedict XVI.