Trial Design: This is an open-label, multi-centre, dose escalation study of ADI-PEG 20 in combination with atezolizumab, pemetrexed and carboplatin in patients with ASS1 deficient, advanced non-squamous NSCLC (stage IIIB/IV). Dose escalation will occur using a 3 + 3 design in an attempt to account for the expected toxicity from the pemetrexed and carboplatin chemotherapy backbone. Patients will initially receive 4 cycles of ADI-PEG 20 in combination with atezolizumab, pemetrexed and carboplatin unless there is documented disease progression or unacceptable toxicities. On completion of 4 cycles and in the absence of disease progression requiring other therapeutic interventions, patients will receive additional cycles of ADI-PEG 20 and atezolizumab for up to a maximum of 2 years. The primary aim of this trial is to determine the maximum tolerated dose (MTD) of ADI-PEG 20 in combination with Atezolizumab, Pemetrexed and Carboplatin in ASS1 deficient advanced non-squamous NSCLC. Secondary objectives include safety and preliminary efficacy signals.