Theo became the Lord of Highgarden in 5 AC after his father, Lord Harlen Tyrell, perished in the First Dornish War. King Aegon I Targaryen flew Balerion to Highgarden to take counsel with the young Warden of the South, but Theo was reluctant to take any further part in the war against Dorne after the fate that had befallen his father. However, House Tyrell eventually became embroiled when the conflict spilled out beyond the Red Mountains.
After the Targaryens made peace with the Dornishmen, Theo spent his time consolidating Tyrell power. He called a council of septons and maesters to examine and dismiss the claims put forth by other houses of the Reach trying to claim Highgarden.