Although there is great interest in the improving the ability to track patients’ change over time in routine clinical care settings, no standardized transdiagnostic measure is currently available for busy clinicians to apply. The Clinical Global Impression (CGI) scales are simple measures widely used as outcomes in psychiatric clinical trials. However, the CGI suffers from poorly defined scoring anchors. Efforts to improve the anchors by enhancing the anchor descriptions have proven useful but are limited by being disease-specific, thereby acting as a barrier to the routine clinical adoption of the CGI. To inform the development of more broadly applicable CGI scoring anchors, we surveyed 24 clinical trial investigators, asking them to rank-order seven elements that inform their CGI-Severity (CGI-S) scoring. Symptom severity emerged as the most important element in determining CGI-S scores; the functional status of the patient emerged as a second element. Less importance was given to self-report symptom scores, staff observations, or side effects. Relative rankings of the elements’ importance did not differ by investigators’ experience nor time usually spent with patients. We integrated these results with published illness-specific CGI anchors to develop the Transdiagnostic CGI (T-CGI), which employs standardized scoring anchors applicable across psychiatric illnesses. Pending validity and reliability evaluations, the T-CGI may prove well-suited for inclusion in routine clinical settings and for incorporation into electronic medical records as a simple and useful measure of treatment efficacy.