From A Wiki of Ice and Fire
|Title|| High Steward of Highgarden|
Lord of Highgarden
Warden of the South
Lord Paramount of the Mander
|Died||In 5 AC, at Dorne|
|Book(s)|| A Game of Thrones (Mentioned)|
The World of Ice and Fire (Mentioned)
When King Mern of the Reach died at the Field of Fire, Harlen Tyrell was but his steward. House Tyrell could claim descent from Garth Greenhand, but only through the female line. When Aegon I descended on Highgarden, Harlen Tyrell bent the knee and pledged fealty. For this, Aegon named him Lord of Highgarden and gave him dominion over the Reach as Lord Paramount of the Mander and Warden of the South.
Aegons judgement proved sound as Harlen served House Targaryen loyally and proved able in his lordship of the Reach, despite the resentment of some of his banner men at having to serve a an "upjumped stewart". While other Houses all had better lineage and close ties to the now extinct House Gardener line, Aegon choose Harlen in part because Harlen had not taken up arms against Aegon and his sisters were as many of the others had.
Harlen was dubious of the Targaryen plan to try and conquer Dorne but nonetheless answered the Conqueror's summons and took part in the First Dornish War. He and his army vanished in 5 AC in the sands of Dorne. Harlen was succeeded by his son Theo Tyrell.