Background: Clinical Practice Guidelines (CPGs) synthesize the best available evidence to guide clinician and patient decision making. There are a multitude of barriers and facilitators to clinicians adhering to CPGs; however, little is known about active cancer treatment CPG adherence specifically. This systematic review sought to identify clinician attitudes, and perceived barriers and facilitators to active cancer treatment CPG adherence. Methods: A systematic search was undertaken of five databases; Ovid Medline, PsychInfo, Embase, Scopus, CINAHL, and PROQUEST. The retrieved abstracts were screened for eligibility against inclusion criteria, and a full text review was conducted of all eligible studies. Data were extracted, and a quality assessment was conducted of all included studies. The qualitative papers were thematically analyzed. Attitudes, barriers, and facilitating factors extracted from the quantitative papers were categorized within the qualitative thematic framework. Results: The search resulted in the identification of 9676 titles. After duplicates were removed, abstracts screened, and full texts reviewed, 15 studies were included. Four themes were identified which related to negative clinician attitudes and barriers to active cancer treatment CPG adherence: (1) concern over CPG content and currency of CPGs; (2) concern about the evidence underpinning CPGs; (3) clinician uncertainty and negative perceptions of CPGs; and (4) organizational and patient factors. The review also identified four themes related to positive attitudes and facilitators to active cancer treatment CPG adherence: (5) CPG accessibility and ease of use; (6) endorsement and dissemination of CPGs and adequate access to treatment facilities and resources; (7) awareness of CPGs and belief in their relevance; and (8) belief that CPGs support decision making, improve patient care, reduce clinical variation, and reduce costs. Conclusion: These results highlight that adherence to active cancer treatment CPG recommendations by oncology clinicians is influenced by multiple factors such as attitudes, practices, and access to resources. The review has also revealed many similarities and differences in the factors associated with general CPG, and active cancer treatment CPG, adherence. These findings will inform tailored implementation strategies to increase adherence to cancer treatment CPGs. Trial registration: PROSPERO (2019) CRD42019125748.