Inclusion Criteria - Cohort Expansion Phase:

          -  Histologically-proven, unresectable, locally advanced or metastatic melanoma or NSCLC

               -  Melanoma: Advanced or metastatic melanoma patients may be systemic therapy naïve
                  or may have received systemic treatment for unresectable locally advanced or
                  metastatic disease. A patient who previously received systemic therapy must have
                  had progression on a checkpoint inhibitor (e.g., anti-PD-L1, anti-PD-1,
                  anti-CTLA-4) as the most recent prior therapy.

               -  NSCLC: NSCLC that has progressed during or following 1 or more prior systemic
                  therapies for unresectable locally advanced or metastatic disease. Patients who
                  are intolerant of, or have refused treatment with standard first line cancer
                  therapy, will be allowed to enroll. Patients must not have had more than 5 prior
                  systemic regimens (excluding experimental therapies) for unresectable locally
                  advanced or metastatic disease.

          -  B7-H3 expression is not required for eligibility in this study; however, tumor
             expression of B7-H3 will be evaluated for all patients.

          -  Measurable disease per RECIST 1.1 criteria

          -  ECOG performance status 0 or 1

          -  Acceptable laboratory parameters and adequate organ reserve.

        Exclusion Criteria - Cohort Expansion Phase:

          -  Patients with a history of symptomatic central nervous system metastases, unless
             treated and asymptomatic

          -  Patients with history of autoimmune disease with certain exceptions

          -  History of allogeneic bone marrow, stem cell, or solid organ transplant

          -  Treatment with systemic cancer therapy or investigational therapy within 4 weeks;
             radiation within 2 weeks; trauma or major surgery within 4 weeks

          -  History of clinically-significant cardiovascular disease; gastrointestinal
             perforation; gastrointestinal bleeding, acute pancreatitis or diverticulitis within 4
             weeks;

          -  Active viral, bacterial, or systemic fungal infection requiring parenteral treatment
             within 7 days; positive for human immunodeficiency virus or AIDS, hepatitis B or C.

          -  Known hypersensitivity to recombinant proteins, polysorbate 80, or any excipient
             contained in the drug or vehicle formulation for MGA271 or ipilimumab.