This study will evaluate the safety, preliminary efficacy, and pharmacokinetics (PK) of momelotinib (MMB) and erlotinib, as well as define the maximum tolerated dose (MTD) of momelotinib (MMB) combined with erlotinib in adults with epidermal growth factor receptor (EGFR)-mutated, EGFR tyrosine kinase inhibitor (TKI) naive metastatic non-small cell lung cancer (NSCLC). Participants will be sequentially enrolled to receive progressively increasing doses of momelotinib (MMB) in combination with erlotinib. Escalation of momelotinib (MMB) doses will proceed to the MTD, defined as the highest tested dose associated with dose-limiting toxicities (DLT) during the first 28 days of combined erlotinib and momelotinib (MMB) treatment. There will be four dose levels and each treatment cycle will consist of 28 days.

Key Inclusion Criteria:

• Metastatic NSCLC with documented EGFR exon 19 deletion or exon 21 (L858R) substitution mutation

• Treatment naive OR one prior standard chemotherapy that is platinum-based

• Adequate organ function defined as follows:

  • Hepatic: Total bilirubin < upper limit of the normal range (ULN); aspartate transaminase (AST) and alanine transaminase (ALT) ≤ 3 x ULN
  • Hematological: absolute neutrophil count (ANC) ≥1500 cells/mm^3, platelet ≥ 100,000 cells/mm^3, hemoglobin ≥ 9.0 g/dL
  • Renal: Serum creatinine < ULN OR calculated creatinine clearance (CLcr) of ≥ 60 ml/min

• Eastern Cooperative Oncology Group (ECOG) Performance Status of 0, 1, or 2

Key Exclusion Criteria:

• Known positive status for human immunodeficiency virus (HIV)
• Chronic active or acute viral hepatitis A, B, or C infection (testing required for hepatitis B and C)
• Presence of > Grade 1 peripheral neuropathy
• Symptomatic leptomeningeal, brain metastases, or spinal cord compression.
• History of interstitial pneumonitis

Note: Other protocol defined Inclusion/Exclusion criteria may apply.