What rank did Mordecai hold at the end of the book of Esther?

Answer: Second only to the king

Mordecai the Jew was second only to King Xerxes, seeking the good of his people and speaking peace to them.

“For Mordecai the Jew was second only to King Xerxes, preeminent among the Jews and highly favored by his many kinsmen, seeking the good of his people and speaking peace to all his countrymen.”— Esther 10:3 (BSB)