He entered the diplomatic service of the Holy See in 1970, serving in the Pontifical Representations in Senegal and Scandinavia, and then in the Council of Public Affairs of the Church of the Vatican Secretariat of State.
He traveled to Latin America in 1978, where he accompanied Antonio Cardinal Samoré in successfully averting war between Chile and Argentina over the Beagle conflict.
[2] Sainz was named nuncio to the Democratic Republic of the Congo on 7 October 1992 (offering the nunciature as a place of relief and refuge to those persecuted during the Rwandan genocide), to the European Community on 22 January 1999,[5] and to Great Britain on 11 December 2004.
[7] On 2 December 2010, a Mass of thanksgiving was celebrated at Westminster Cathedral, which marked the end of his role as papal nuncio to Great Britain.
[8] Archbishop Sainz Muñoz was diagnosed with an aggressive brain tumour, and returned to his native Spain, where he received his chemotherapy.