Cecily McMillan didn't come from much: she had a hardscrabble life bouncing between the members of a broken family scattered from Texas to Atlanta. Her relationship with her parents became increasingly strained, and at sixteen she was legally emancipated and taken in by a beloved teacher. She became politically active at a young age, leading walkouts against the Iraq War in high school and protesting against union busting in college. When she moved to New York for a masters degree at the New School, she found herself planning what would later become Occupy Wall Street.

On St. Patrick's...