Bryan Stevenson is the founder of the Equal Justice Initiative in Montgomery, Alabama, an organization which gives legal representation to people from marginalized social groups who have been wrongfully convicted or whose sentences are unreasonably harsh. In Just Mercy, he writes about his career, with a focus on one particular prisoner who became a close friend, Walter MacMillian.

MacMillian had been a successful businessman before he was convicted of murder and sentenced to death. The accusation of murder has originally come from a man named Ralph Myers, a mentally unstable associate of MacMillian's former girlfriend, and had been supported by an openly racist sheriff. It was clear that both these men were acting vindictively, and the trial also featured suppression of evidence and bribed witnesses. MacMillian was sentenced to death, and served six years on death row before Stevenson was able to secure his release. The trauma he suffered in prison ruined MacMillian's life, leaving him with dementia and severe anxiety.

While his friendship with MacMillian did not change the course of Stevenson's life, it did confirm the value of what he was doing, and the scale of the problem the Equal Justice Initiative had been founded to fight. This personal relationship gave a human face to the suffering and injustice which might otherwise have been somewhat abstract. MacMillian was also a remarkably generous and kindly person, and the experience of seeing a man like this destroyed by a corrupt system redoubled Stevenson's sense of purpose. At MacMillian's funeral, Stevenson spoke of how much his client and friend had taught him about resilience, forgiveness, and hope.