Studies on therapist factors have mostly focused on therapist traits rather than states such as affect. Research related to therapist affect has often looked at therapist baseline well-being or therapist reactions, but not both. Fifteen therapists and 51 clients rated pre- and postsession affect, as well as postsession working alliance and session quality, for 1,172 sessions of individual psychotherapy at a community clinic. Therapists’ affect became more positive when clients were initially positive and when clients became more positive over the session, and became more negative when clients were initially negative and when clients became more negative over the session. Furthermore, when therapists were initially positive in affect and when therapists became more positive over the session, clients rated the session quality to be high. Conversely, when therapists were initially negative in affect and when therapists became more negative over the session, clients rated the session quality and working alliance low. On open-ended questions, therapists reported mood shifts in 67% of sessions (63% positive, 50% negative). Positive affect change was attributed to collaborating with the client, perceiving the client to be engaged, or being a good therapist. Negative affect change was attributed to having a difficult client, perceiving the client to be in distress, or being a poor therapist. Thus, therapist state affect at presession and change in affect across a session may independently contribute to the process and outcome of therapy sessions. The examination of within-therapist variables over the course of therapy may further our understanding of therapist factors.