One of the themes of Manchild in the Promised Land is the persistence of racism in the urban ghettos of the north after the Great Migration. Sonny (the main character, who is based on Claude Brown) is born in Harlem shortly after his parents have left the South during the Great Migration in the hope of a better life in the North. However, what they find in Harlem is poverty, crime, and limited opportunities. It is for these reasons that gangs run rampant in Harlem, and it is only through a combination of fortitude and diligence that Sonny is able to leave the ghetto for a better life.

Another theme is the importance of mentors. Sonny finds his parents unable to guide him through his childhood and adolescence. His mother has a resigned personality, and his father is indifferent to his children and their lot. It is only by chance that Sonny meets Mr. Papanek at a boys' reform school, and Sonny realizes that Papanek really cares for him. He eventually listens to Papanek's advice to become educated. Before that point, Sonny is guided by the older boys around him, such as Danny, who have turned to a life of drugs and crime. Having mentors and guidance is critical to Sonny.

The book also emphasizes the way in which the urban ghetto breeds hopelessness and kills young people's dreams. Sonny is the exception, the one person who can escape to pursue a better life. His childhood friend Sugar is not so lucky, as she is caught up in a nexus of prostitution and drugs. It is mostly by chance that Sonny does not succumb to this fate, as heroin causes him to get sick the first time he uses it. Most of the characters in the book cannot escape the poverty and hopelessness around them.