Stanley is in his late thirties and has been the only guest at the boarding house for a year. He is the play’s protagonist and undergoes the central journey of the play as the subject of the titular birthday party.

However, he is an unconventional protagonist, because his actions that drive the play are weak, fading quickly into inaction. During his year at the boarding house, Stanley has isolated himself in his room, plagued by ambiguous anxieties. He describes a derailed prior life as a concert pianist, but he gave up trying when he showed up to perform and found the performance space closed.

Stanley has given up on himself and his unfulfilled potential. He is unshaven and sloppy in appearance, only coming downstairs to eat because Meg coerces him, and he admits that his appearance is rough because he has had a hard year of too much drinking. Stanley resents Meg but also gives in to her as she turns him into her surrogate son, fabricates his identity (and birthday), and tells him not to leave. Above all else, Stanley is trapped. He makes some attempts to fight back against Goldberg and McCann or to leave, but as he tells Lulu, there is nowhere to go.