|Allegiance||Faith of the Seven|
|Book(s)||Fire & Blood (mentioned)|
Mattheus was described as a great whale of a man, famed as much for his corpulence as for the magnificence of his robes. 
Septon Mattheus claimed to be a descendant of the Gardener Kings who once ruled the Reach. In 49 AC many felt he was almost guaranteed to be the next High Septon. He became a part of the court of King's Landing during the regency of the young King Jaehaerys I Targaryen. Septon Mattheus accompanied Rogar Baratheon and Alyssa Velaryon to Dragonstone when the small council learned that King Jaehaerys and his younger sister, Alysanne had fled to the island fortress to wed. The incestuous marriage enraged him and he threw insults at the couple. Jaehaerys would not allow his bride to be insulted and ordered Mattheus to be silent or he would have his lips sewn shut.
After the death of the High Lickspittle, Mattheus became a likely candidate to become his successor. During the first of the four ballots required for the election, Mattheus was the lead candidate, but his supporters dwindled there after with each ballot. On the fourth ballot, Septon Alfyn was elected.