We meet Mayor Seuly twice during Conrad's Fate. During his first appearance, he is part of the group that meets at the Magicians' Circle hosted by Conrad's uncle, Alfred Grant. Present at these sessions are various town bigshots. Conrad is impressed by their clothes, their expensive airs, and their sophistication. They drink expensive port, smoke cigars, and trade news about threats to their various holdings. Mayor Seuly, for example, owns the ironworks at the other end of town. At this particular meeting, they all discuss Conrad's karma, and make suggestions as to how he can use magic to disable whoever it is up at Stallery Manor who makes profits that they themselves are unable to make.
These wealthy men all appear to be magic users, since they suggest specific techniques Conrad might use at the Manor to kill the unknown person. The only contribution Uncle Alfred makes is to remind them that Conrad doesn't know any magic. So Mayor Seuly suggests they have Conrad summon a Walker. He gives Conrad the basic instructions, selects a wine cork as a ordinary item unlikely to be noticed, and leads the group in various steps to prepare the cork. And Conrad takes it away, planning to follow those instructions to a T.
The next time the Mayor appears, however, is as a guest at Stallery. After all of Conrad's experiences with actual aristocrats, Mayor Seuly looks kind of shrunken and pathetic in his expensive clothes. He seems to be making schemes for feathering his own nest, rather than pursuing the 'one for all, all for one' approach he seemed to be taking previously. And what happens to him at the end of the book, although not what he thought would happen, is an interesting ending.