• Objective response rate (ORR) [ Time Frame: Through study completion, up to approximately 31 months ]
  Defined as the percentage of participants having complete response (CR) or partial response (PR) according to Response Evaluation Criteria in Solid Tumors (RECIST) v1.1 as determined by investigator assessment.
• Duration of response (DOR) [ Time Frame: Through study completion, up to approximately 31 months ]
  Defined as the time from an initial objective response (CR or PR) according to RECIST v1.1 until disease progression as determined by investigator assessment or death due to any cause.
• Disease control rate (DCR) [ Time Frame: Through study completion, up to approximately 31 months ]
  Defined as the number of participants with CR or PR as best response or stable disease that was maintained for at least 12 weeks.
• Cmax of INCMGA00012 when given in combination with chemotherapy agents [ Time Frame: Through post-induction Cycle 5 Day 1, up to 15 weeks ]
  Maximum observed plasma or serum concentration.
• Tmax of INCMGA00012 when given in combination with chemotherapy agents [ Time Frame: Through post-induction Cycle 5 Day 1, up to 15 weeks ]
  Time to maximum concentration.
• Cmin of INCMGA00012 when given in combination with chemotherapy agents [ Time Frame: Through post-induction Cycle 5 Day 1, up to 15 weeks ]
  Minimum observed plasma or serum concentration over the dose interval.
• AUC0-t of INCMGA00012 when given in combination with chemotherapy agents [ Time Frame: Through post-induction Cycle 5 Day 1, up to 15 weeks ]
  Area under the plasma or serum concentration-time curve from time = 0 to the last measurable concentration at time = t.

• Advanced and/or Metastatic Solid Tumors
• Stage IIIB Not Amenable to Curative Therapy to Stage IV Non-small Cell Lung Cancer
• Advanced/Metastatic Unresectable Malignant Pleural Mesothelioma

• Drug: Retifanlimab
  INCMGA00012 administered intravenously every 3 weeks.
  Other Name: INCMGA00012
• Drug: Gemcitabine
  Gemcitabine administered intravenously on Days 1 and 8 of 21-day cycles.
• Drug: Cisplatin
  Cisplatin administered intravenously on Day 1 of 21-day cycles.
• Drug: Pemetrexed
  Pemetrexed administered intravenously on Day 1 of 21-day cycles.
• Drug: Carboplatin
  Carboplatin AUC5 or AUC6 administered intravenously on Day 1 of 21-day cycles.
• Drug: Paclitaxel
  Paclitaxel administered intravenously on Day 1 of 21-day cycles.

• Experimental: INCMGA00012 + gemcitabine/cisplatin
  Interventions:

  • Drug: Retifanlimab
  • Drug: Gemcitabine
  • Drug: Cisplatin
• Experimental: INCMGA00012 + pemetrexed/cisplatin
  Interventions:

  • Drug: Retifanlimab
  • Drug: Cisplatin
  • Drug: Pemetrexed
• Experimental: INCMGA00012 + pemetrexed/carboplatin
  Interventions:

  • Drug: Retifanlimab
  • Drug: Pemetrexed
  • Drug: Carboplatin
• Experimental: INCMGA00012 + paclitaxel/carboplatin
  Interventions:

  • Drug: Retifanlimab
  • Drug: Carboplatin
  • Drug: Paclitaxel

Inclusion Criteria:

• Advanced and/or metastatic solid tumors including the following: histologically or cytologically confirmed diagnosis of Stage IIIB not amenable to curative treatment or Stage IV non-small cell lung cancer (pemetrexed-platinum treatment groups must have nonsquamous histology type); and histologically or cytologically confirmed diagnosis of advanced/metastatic unresectable malignant pleural mesothelioma.
• No prior systemic treatment with the following exceptions: participants with a known sensitizing mutation (eg, BRAF, EGFR, ALK, or ROS1) should have had disease progression on or following an approved targeted tyrosine kinase inhibitor; and participants who received adjuvant or neoadjuvant chemotherapy are eligible if the adjuvant/neoadjuvant therapy was completed at least 6 months before the date of enrollment.
• Measurable or nonmeasurable tumor lesions per RECIST v1.1.
• Eastern Cooperative Oncology Group performance status 0 to 1.

Exclusion Criteria:

• Received prior treatment with checkpoint inhibitor agents (such as anti-PD-1, anti-PD-L1, anti-PD-L2, or anti-CTLA-4).
• Had major surgery within 3 weeks before the first dose of study treatment.
• Received radiation therapy to the lung(s) that is > 30 Gy within 6 months of the first dose of study treatment.
• Received palliative radiotherapy within 7 days before the first dose of study treatment.
• Has ≥ Grade 2 residual toxicities from the most recent prior therapy (except alopecia).
• Organ function (renal, hepatic), bone marrow reserve, and coagulation panel outside the protocol-defined laboratory values.
• Is currently participating and receiving investigational therapy or has participated in a study of an investigational agent and received study therapy or used an investigational device within 4 weeks before the first dose of study treatment.
• Has active autoimmune disease requiring systemic immunosuppression with corticosteroids (> 10 mg once daily of prednisone or equivalent) or immunosuppressive drugs within 2 years before the first dose of study treatment.
• Is on chronic systemic steroids (> 10 mg once daily of prednisone or equivalent).
• Known active central nervous system metastases and/or carcinomatous meningitis (patients with previously-treated and clinically stable brain metastases are eligible and a washout period of ≥ 4 weeks since radiation therapy is required).
• Known additional malignancy that is progressing or requires active treatment.
• Evidence of interstitial lung disease or active, noninfectious pneumonitis.
• History of organ transplant, including allogeneic stem cell transplantation.
• Active infections requiring systemic antibiotics.
• Known active hepatitis B or C.
• Has a diagnosis of immunodeficiency, including participants known to be HIV positive (positive for HIV 1/2 antibodies).
• Significant cardiac event within 6 months before Cycle 1 Day 1.
• Has received a live vaccine within 28 days of the planned start of study treatment.
• Known hypersensitivity to any component of the study drugs, excipients, or another monoclonal antibody which cannot be controlled with standard measures (eg, antihistamines and corticosteroids).