Heim's diplomatic career began in January 1947, when he was assigned to the Apostolic Nunciature in Paris to become personal secretary to Archbishop Angelo Roncalli, the future Pope John XXIII.

Heim stayed in Paris for four years, during which time the two of them laid the foundations for a renaissance of heraldry in the Roman Catholic Church.

When Pope John Paul II visited Great Britain in 1982, the United Kingdom and the Vatican had established full diplomatic relations, and Heim became the Apostolic Pro-Nuncio, the Vatican's first fully-fledged ambassador to the Court of St. James's since the Reformation.

[3] When Heim was sent to Vienna in 1951 he maintained close contact with Archbishop Roncalli, who commissioned him to design his new coat of arms as Patriarch of Venice.

When Roncalli was elected Pope in 1958, he asked Heim to design his personal papal coat of arms.

[3] Archbishop Heim served as patron of Cambridge University Heraldic and Genealogical Society from 1980 until his death in 2003 at the age of 92 in Olten.