Who was the first ambassador in Islam, and what were his most prominent actions and where did they take place?

The first ambassador in Islam was Mus'ab ibn 'Umayr. The Prophet, peace and blessings be upon him, sent him to Medina after the First Pledge of Aqabah, to teach its people the Qur'an, instruct them in Islam, and deepen their understanding of the religion. Thus, he was called "the Reciter" (al-Muqri'). Among his most notable achievements was the conversion of several prominent Ansar leaders, such as Usaid ibn Hudair and Sa'd ibn Mu'adh, through his efforts.