You would normally introduce an ambassador or high commissioner as "His/Her Excellency." For example, if the ambassador's name was John Smith, he would be addressed "His Excellency, John Smith." In writing, this title can be abbreviated as "HE." In the case of the Apostolic Nuncio (the representative of the Vatican) you would introduce him as "His Most Reverend Excellency."
This form of formal and respectful address is called an honorific and variants of this would be used when addressing monarchs, judges, Members of Parliament, cabinet ministers and Prime Ministers, as well as religious leaders.
Whenever you introduce an ambassador at a formal event--such as when you are inviting him/her to give a speech--you should use the appropriate honorific. If you are speaking with the ambassador in private, or even at a dinner party or formal event, you can simply call him "Mr./Ms.," in lieu of the honorific.
This form of formal and respectful address is called an honorific and variants of this would be used when addressing monarchs, judges, Members of Parliament, cabinet ministers and Prime Ministers, as well as religious leaders.
Whenever you introduce an ambassador at a formal event--such as when you are inviting him/her to give a speech--you should use the appropriate honorific. If you are speaking with the ambassador in private, or even at a dinner party or formal event, you can simply call him "Mr./Ms.," in lieu of the honorific.