Introduction The T790M resistance mutation is present in about one-half of epidermal growth factor receptor (EGFR)-positive advanced non-small cell lung cancer (NSCLC) patients at disease progression. We aimed to assess the prevalence of this mutation in a real-world setting and the clinical impact of repeated biopsies in its detection. Methods This was a single-center retrospective cohort study of patients with EGFR-positive advanced NSCLC diagnosed between 2016 and 2018, who experienced radiographic disease progression during first-line treatment with first- or second-generation EGFR-tyrosine kinase inhibitor (TKI). The frequency of T790M detection and the number of rebiopsies were determined. Results A total of 88 patients were included, with a median age of 65 years (range: 38-84 years). The majority of the participants were females (63 (72%)) and non-smokers (70 (81%)). Upon disease progression, 80 (91%) patients were tested for T790M mutation, and the resistance mutation was detected in 57 (71%) cases (58% in plasma samples and 42% in tissue/cytology samples). In 14 (25%) cases, T790M mutation was only detected after rebiopsy (57% by liquid biopsy), which increased the rate of mutation detection in 17%. Subsequent treatment with third-generation EGFR-TKI was possible in 42 (74%) of T790M-positive cases. Detection of T790M mutation was more likely in patients who were less than 65 years old, with EGFR exon 19 deletions and duration of first-line treatment of more than 12 months (p < 0.05). Conclusions The frequency of T790M mutation in this study was higher than previously reported, suggesting that repeated biopsies after a negative result are beneficial. This allowed a greater percentage of patients to receive sequential osimertinib in our clinical practice.