What is the simple subject of this sentence? Then, glancing upward at Lady Weather's face, he saw a look of consternation.

The simple subject is "he".

The simple subject of the sentence "Then, glancing upward at Lady Weather's face, he saw a look of consternation" is "he".

A simple subject is one word that can have modifiers before or after it. A basic sentence consists of a subject and a predicate, or a subject and a verb. In our case, the modifier before the subject "he" is

"then, glancing upward at Lady Weather's face"; while, the modifier after the subject is "saw a look of consternation."