Mateos worked with BYU OC Jeff Grimes at LSU for one season.
Bodie Reeder replaces Graham Harrell, who left for the same job at USC.
Ansley will also take over playcalling duties from the head coach.
Seminoles were 129th out of 130 teams in yards per rushing attempt, were 16th worst in sacks allowed.
Jeffrey Gunter had also considered NC State, North Carolina and Oregon.