Protein-losing nephropathy (PLN) involves a heterogeneous group of pathologies leading to selective glomerular damage and development of renal disease. ICGN, the main cause of PLN, requires immunosuppressive treatment. However, the scientific evidence in veterinary medicine on immunosuppressive therapeutic schemes in this condition is limited. The aim of this study is to describe the clinical and paraclinical evolution of five dogs with PLN, presumably associated with ICGN, treated with chlorambucil as immunosuppressive monotherapy. Suspected IGCN was established by the presence of a urine protein–creatinine ratio (UPC) ≥ 3 without response to standard therapy, hypoalbuminemia < 2, or progressive azotemia. Patients were treated with a dosage range of chlorambucil from 0.16 to 0.4 mg/kg (mean 0.25 mg/kg) every 24 h as the sole immunosuppressant. In the end, 4/5 patients showed significant clinical improvement, 3/3 had resolution of the nephrotic syndrome, 5/5 had a sustained decrease in UPC values during follow-up and no relevant adverse effects were observed. In this report, chlorambucil proved to be a well-tolerated and potentially effective monotherapy for immune-mediated PLN in dogs.