Previous studies have shown that the use of therapy skills in between sessions is an important mechanism of symptom improvement. The current study expands this line of research by using a diary approach to examine the use of therapy skills in daily life. A sample of 39 depressed adults (85% female, mean age 38) were signaled twice per week throughout the course of either cognitive-behavioral therapy (CBT) or self-system therapy (SST). Results showed that, on days when participants reported greater use of therapy skills, they reported better mood and functioning in almost all domains. Additionally, participants in CBT reported greater use of cognitive skills while participants in SST reported greater use of self-regulatory skills. This study demonstrates that repeated assessments of daily events and experiences, which prevent retrospective reporting biases, further confirm the importance of therapy skill use as a mechanism of action in psychotherapy.