Background The neutrophil-to-lymphocyte ratio (NLR) is a measure of systemic inflammation and a prognostic factor for multiple malignancies. This study assesses the value of the NLR as an independent prognostic marker in triple-negative breast cancer (TNBC) and explores the association between dynamic NLR changes and patient outcomes. Methods The study retrospectively analyzed a prospectively maintained database including patients 18 to 80 years old with TNBC treated at the authors’ institution between 2006 to 2016. Clinical and demographic data were collected, including blood test results and treatments received. Age at diagnosis, stage of disease, and NLR scores were tested for association with overall and disease-free survival in uni- and multivariate Cox models. Results The inclusion criteria were met by 329 women with a median age of 58. Most of the patients had early-stage disease (30.1% with stage 1 and 47% with stage 2 malignancy). An NLR higher than 2.84 at diagnosis was associated with decreased overall survival (hazard ratio HR, 1.8; 95% confidence interval CI, 1.023–3.176), whereas an NLR higher than 7.82 at any time during the follow-up period was a strong predictor of 5-year mortality (HR, 10.76; 95% CI, 4.193–26.58), independent of age or stage of disease. Patients who experienced recurrence had a higher NLR than their counterparts during the 6 months before recurrence. The NLR also significantly rose during the final 18 months of life ( p  < 0.01). Conclusion The NLR is an important prognostic marker in TNBC, both at diagnosis and during the course of the disease. Moreover, dynamic changes in NLR strongly correlate with disease recurrence and the time of death.