- Has predominantly squamous cell histology NSCLC. - Is currently participating and receiving study therapy or has participated in a study of an investigational agent and received study therapy or used an investigational device within 4 weeks prior to administration of pembrolizumab. - Before the first dose of study medication: a) Has received prior systemic cytotoxic chemotherapy for metastatic disease, b) Has received antineoplastic biological therapy (e.g., erlotinib, crizotinib, cetuximab), c) Had major surgery (<3 weeks prior to first dose) - Received radiation therapy to the lung that is >30 Gray (Gy) within 6 months of the first dose of study medication. - Completed palliative radiotherapy within 7 days of the first dose of study medication. - Is expected to require any other form of antineoplastic therapy while on study. - Received a live-virus vaccination within 30 days of planned start of study medication. - Has clinically active diverticulitis, intra-abdominal abscess, gastrointestinal obstruction, peritoneal carcinomatosis. - Known history of prior malignancy except if participant has undergone potentially curative therapy with no evidence of that disease recurrence for 5 years since initiation of that therapy, except for successful definitive resection of basal cell carcinoma of the skin, superficial bladder cancer, squamous cell carcinoma of the skin, in situ cervical cancer, or other in situ cancers. - Has known active central nervous system (CNS) metastases and/or carcinomatous meningitis. - Previously had a severe hypersensitivity reaction to treatment with another monoclonal antibody (mAb). - Known sensitivity to any component of cisplatin, carboplatin or pemetrexed. - Has active autoimmune disease that has required systemic treatment in past 2 years. - Is on chronic systemic steroids. - Is unable to interrupt aspirin or other nonsteroidal anti-inflammatory drugs (NSAIDs), other than an aspirin dose ≤1.3 g per day, for a 5-day period (8-day period for long-acting agents, such as piroxicam). - Is unable or unwilling to take folic acid or vitamin B12 supplementation. - Had prior treatment with any other anti-programmed cell death-1 (PD-1), or PD-ligand 1 (PD-L1) or PD-L2 agent or an antibody targeting other immuno-regulatory receptors or mechanisms. Has participated in any other pembrolizumab study and has been treated with pembrolizumab. - Has an active infection requiring therapy. - Has known history of Human Immunodeficiency Virus (HIV). - Has known active Hepatitis B or C. - Has known psychiatric or substance abuse disorder that would interfere with cooperation with the requirements of the trial. -Is a regular user (including "recreational use") of any illicit drugs or had a recent history (within the last year) of substance abuse (including alcohol). - Has symptomatic ascites or pleural effusion. - Has interstitial lung disease or a history of pneumonitis that required oral of IV glucocorticoids to assist with management. -Is pregnant or breastfeeding, or expecting to conceive or father children prior to 120 days after the last dose of study medication or through 180 days after last dose of chemotherapeutic agents.