Appearance and Character
Mace is handsome and a once-powerful-looking man, but has since turned to fat. He has curly brown hair and cuts his beard into a triangular shape that has white spots within it.
Mace is a prematurely old and tedious man lacking in political savvy. His sole military accomplishment was the siege of Storm's End. Though he has tried to use the Battle of Ashford as evidence of strategic brilliance, he actually had little to do with the fighting as the battle was won by his vanguard, led by Randyll Tarly, before his main force entered the field.
War of the Five Kings
Mace Tyrell marched with Renly Baratheon and their armies from Highgarden to King's Landing. There the Lord of the Reach led the siege of the city, while his sons Garlan and Loras accompanied Renly Baratheon and Randyll Tarly west to face Tywin Lannister.
After the defeat at the Battle of the God's Eye, Renly and his surviving forces retreated to King's Landing. Mace urged retreat, stating that Tywin was obviouosly very cunning, and would not strike if he were not confident in victory. Renly, however, would hear none of it, and engaged his brother's father-in-law in the Battle for King's Landing.
King Renly emerged victorious, defeating the Westerlands and taking the city in the same day. However he had only possessed it for about a week before word arrived of Ironborn raiding in the Reach. Worried for his homeland, and the largely undefended strongholds in it, Mace petitioned the king for permission to return and defend it. This was granted, and he turned his men back around.
Most of the battles against the Ironborn were led by different Lords, with Mace returning to Highgarden and nominally leading the defence from there.