Introduction: We investigated the effectiveness of lenvatinib (LEN) after disease progression following first-line treatment with atezolizumab plus bevacizumab (Atez/Bev) in patients with unresectable hepatocellular carcinoma (HCC). Methods: One hundred and ten HCC patients treated with Atez/Bev as first-line systemic chemotherapy were enrolled and underwent dynamic computerized tomography/magnetic resonance imaging to determine the treatment response. We evaluated the treatment efficacy and prognosis after second-line LEN treatment, especially in elderly patients. Results: Of the 110 study patients, 88 patients (80%) were determined to have progressive disease (PD) during the observation periods, and 40 patients received second-line LEN therapy. The 40 patients included 13 patients who were unable to continue LEN until the initial evaluation due to adverse events (AE). The 27 patients who were able to continue LEN therapy (Group A) were significantly younger at initiation of Atez/Bev than those who could not continue (71 vs. 77 years old, p = 0.013). Comparing the OS and PFS between Group A and 44 patients that included 13 patients who were unable to continue LEN and 31 patients did not receive the second-line treatment (Group B), the former had significantly better OS than Group B (31.1 vs. 17.8 months, p = 0.035). Conclusions: The tolerability of second-line LEN therapy was lower in elderly patients, and the OS from the start of Atez/Bev therapy was different depending on the tolerability of second-line LEN therapy.