* The patient must have signed and dated an Institutional Review Board (IRB)-approved consent form that conforms to federal and institutional guidelines * Age \>= 18 years * Eastern Cooperative Oncology Group (ECOG) performance status of 0, 1 or 2 * Diagnosis of metastatic adenocarcinoma of colon or rectum without previous chemotherapy or any other systemic therapy for metastatic colorectal cancer except for one cycle of FOLFOX or capecitabine and oxaliplatin (CAPOX), either with or without bevacizumab prior to enrollment. Upon enrollment, the preceding single cycle of FOLFOX or FOLFOX + bevacizumab, if the patient received one, will not count towards patients' assessments per protocol. Cycle 1 day 1 (C1D1) of atezolizumab or C1D1 of mFOLFOX6/bevacizumab + atezolizumab will correspond to the first day the patient received therapy on trial * Tumor determined to be mismatch-repair deficient (dMMR) by Clinical Laboratory Improvement Act (CLIA)-certified immunohistochemical (IHC) assay with a panel of all four IHC markers, including MLH1, MSH2, PMS2, and MSH6; alternatively, MSI-H diagnosed by polymerase chain reaction (PCR)-based assessment of microsatellite alterations (either Bethesda markers or Pentaplex panel) or by next-generation sequencing (NGS) are eligible * Documentation by PET/CT scan, CT scan, or MRI that the patient has measurable metastatic disease per RECIST 1.1 * No immediate need for surgical intervention for the primary tumor or palliative diversion/bypass * Absolute neutrophil count (ANC) must be \>= 1500/mm\^3 (obtained within 28 days prior randomization) * Platelet count must be \>= 100,000/mm\^3 (obtained within 28 days prior randomization) * Hemoglobin must be \>= 8 g/dL (obtained within 28 days prior randomization) * Total bilirubin must be =\< 4 x ULN (upper limit of normal) (obtained within 28 days prior randomization); and * Aspartate aminotransferase (AST) and alanine aminotransferase (ALT) must be =\< 3 x ULN for the lab with the following exception: for patients with documented liver metastases, AST and ALT must be =\< 5 x ULN (obtained within 28 days prior randomization) * Calculated creatinine clearance \>= 30 mL/min (obtained within 28 days prior randomization) * A urine sample tested for proteinuria by either the dipstick method, urinalysis (UA), or a urine protein creatinine (UPC) ratio: * The dipstick method must indicate 0-1+ protein; if dipstick reading is \>= 2+, a 24-hour urine must be done and it must demonstrate \< 1.0 g of protein per 24 hours or a UPC ratio \< 1.0 * A urine protein creatinine (UPC) ratio must be \< 1.0; if the UPC ratio is \>= 1.0 a 24-hour urine must be done and it must demonstrate \< 1.0 g of protein per 24 hours * Urinalysis must indicate \< 30 mg/dl. If urinalysis \>= 30 mg/dl, a 24-hour urine must be done and it must demonstrate \< 1.0 g of protein per 24 hours or a UPC ratio \< 1.0 * International normalized ratio of prothrombin time (INR) and prothrombin time (PT) must be =\< 1.5 x ULN for the lab within 28 days before randomization; patients who are therapeutically treated with an agent such as warfarin may participate if they are on a stable dose and no underlying abnormality in coagulation parameters exists per medical history, regardless of PT/INR results * Pregnancy test done within 28 days prior randomization must be negative (for women of childbearing potential only); pregnancy testing should be performed according to institutional standards; administration of atezolizumab or mFOLFOX6/bevacizumab/atezolizumab may have an adverse effect on pregnancy and poses a risk to the human fetus, including embryo-lethality; should a woman become pregnant or suspect she is pregnant while she or her partner is participating in this study, she should inform her treating physician immediately * Women of child-bearing potential and men must agree to use adequate contraception methods that result in a failure rate of \< 1% per year during the treatment period (hormonal or barrier method of birth control; abstinence) prior to study entry, for the duration of study participation, and for 5 months (150 days) after the last dose of atezolizumab, 6 months after the last dose of bevacizumab, and 6 months after the last dose of mFOLFOX6; a woman is considered to be of childbearing potential if she is not postmenopausal, has not reached a postmenopausal state (\>= 12 continuous months of amenorrhea with no identified cause other than menopause), and has not undergone surgical sterilization (removal of ovaries and/or uterus); examples of contraceptive methods with a failure rate of \< 1% per year include: bilateral tubal ligation; male partner sterilization; intrauterine devices; the reliability of sexual abstinence should be evaluated in relation to the duration of the clinical study and the preferred and usual lifestyle of the patient; periodic abstinence (e.g., calendar, ovulation, symptothermal, or postovulation methods) and withdrawal are not acceptable methods of contraception; men must refrain from donating sperm during this same period