Background: The purpose was to investigate the treatment flow of patients with HER2-positive metastatic breast cancer (mBC), progression-free survival (PFS) and overall survival (OS) across treatment lines and adherence to guidelines (defined as trastuzumab, pertuzumab and chemotherapy first line, where 85% received vinorelbine as backbone and T-DM1 second line). Furthermore, we identified clinical markers to predict the risk of developing brain metastases. Material and Methods: Patients with HER2-positive mBC, diagnosed between 01.01.2014–31.12.2019, registered in the database of the Danish Breast Cancer Group were included in this real-word study. Clinical follow-up was assessed until 01.10.2020 and complete follow-up for overall survival until 01.10.2021. Survival data were analyzed using the Kaplan-Meier method with guidelines adherence analyzed as a time-varying covariate, and the risk of CNS metastasis was estimated by the cumulative incidence function. Results: 631 patients were included. 329 (52%) patients followed the guidelines. The median OS for all patients was 42.3 months (95% Cl, 38.2–48.4), and significantly higher for the patients who followed guidelines; NA (95% CI, 78.2–NA). The median PFS for all patients was 13.4 months (95% Cl, 12.1–14.8), 6.6 (95% Cl, 5.8–7.6) and 5.8 (95% Cl, 4.9–6.9) for first, second and third line of treatment, respectively. Patients with ER-negative mBC had a higher risk of developing brain metastases and patients with high tumor burden had a higher risk of developing brain metastases with an adjusted HR of 0.69 (95% CI, 0.49–0.98), p = 0.047 and 2.69 (95% CI, 1.45–5.00), p = 0.002, respectively. Conclusion: We found that only half of the patients with HER2-positive mBC, received first and second-line treatment according to national guidelines. Patients receiving treatment according to guidelines had a significantly higher median OS compared to patients who did not. We also found that patients with ER-negative disease or high tumor burden had a significantly higher risk of developing brain metastases.