Guido del Mestri was born in Banja Luka in Bosnia and Herzegovina, then under Austro-Hungarian rule, on 13 January 1911 to an Italian father, Count Gian Vito Del Mestri, and an Austrian mother, Baroness Marianna de Grazia.

[2] He was expelled from Romania with the rest of the nunciature personnel in July 1950 when the country came under Communist rule.

[4] On 21 September 1959, Pope John XXIII named him Apostolic Delegate to British Eastern and Western Africa.

[7] Pope John named him titular archbishop of Tuscamia on 28 October 1961[8] and he received his episcopal consecration in Nairobi on 31 December[2] from Cardinal Laurean Rugambwa.

[3] In January 1989, Pope John Paul made him a member of the Congregation for the Evangelization of Peoples and the Council for the Public Affairs of the Church.