• Participant must have been previously diagnosed with histologically or cytologically confirmed unresectable or metastatic adenocarcinoma of the colon or rectum

• Participant must have tumor previously characterized as having wild-type Kirsten rat sarcoma viral oncogene (KRAS), neuroblastoma RAS viral oncogene homolog (NRAS), v-raf murine sarcoma viral oncogene homolog B (BRAF), and without evidence of Erb-b2 receptor tyrosine kinase 2/human epidermal growth factor receptor 2 (ERBB2/HER2) amplification. Additional cohort-specific requirements:

‣ Phase (Ph) 2 (Cohorts A, B, and C) Amivantamab monotherapy: Participant must have received at least 2 but not more than 3 prior lines of systemic therapy in the metastatic setting. Participant must have been diagnosed with left-sided colorectal cancer (CRC) (Cohort A and B) and right-sided (Cohort C)and have received or been intolerant to standard of care (SoC) fluoropyrimidine-, oxaliplatin-, and irinotecan-based chemotherapy and an anti-vascular endothelial growth factor (VEGF) treatment. Participant must be anti-EGFR treatment naive in Cohort A, an anti-epidermal growth factor receptor (EGFR) treatment Cohort B, with or without an anti-EGFR treatment in Cohort C

⁃ Ph 1b Dose Confirmation Cohorts (Ph1b-D and Ph1b-E), Ph2 (Cohorts D and E) Amivantamab+mFOLFOX6/FOLFIRI: Participant must been diagnosed with CRC and have received no more than 1 prior line of systemic therapy in the metastatic setting. Cohort Ph1b-D/Cohort D: Participant must be anti-EGFR treatment naïve, have not received oxaliplatin-based chemotherapy in the metastatic setting, and be eligible for treatment with mFOLFOX6 according to local regulatory approvals and SoC guidelines. Cohort Ph1b-E/Cohort E: Participant must be anti-EGFR treatment naïve, have not received irinotecan-based chemotherapy in the metastatic setting, and be eligible for treatment with FOLFIRI according to local regulatory approvals and SoC guidelines

⁃ Ph2 Cohorts F Amivantamab subcutaneous (SC) + mFOLFOX6: Participants must be treatment-naive for right-sided unresectable or metastatic CRC and be eligible for treatment with mFOLFOX6 according to local regulatory approvals and SoC guidelines

• For Phase 1 dose confirmation cohorts (Cohorts Ph1b-D and Ph1b-E): Participant must have evaluable disease. For Phase 2: Participant must have measurable disease according to Response Criteria in Solid Tumors (RECIST) Version 1.1. If only one measurable lesion exists, it may be used for the screening biopsy as long as baseline tumor assessment scans are performed greater than or equal to (\>=) 7 days after the biopsy

• Participant must have Eastern Cooperative Oncology Group (ECOG) performance status (PS) 0 or 1

• Participant must have a tumor lesion amenable for biopsy and agree to mandatory protocol-defined screening biopsy. Biopsies are required if clinically feasible for participants in Ph1b-D, Ph1b-E, and Cohort F. For Cohort F, archival tissue is required if a fresh biopsy is not feasible

• A female participant of childbearing potential must have a negative serum pregnancy test at screening and within 72 hours of the first dose of study treatment and must agree to further serum or urine pregnancy tests during the study. Note: Participant must not be pregnant, breastfeeding, or planning to become pregnant while enrolled in this study