Inclusion Criteria:  Aged at least 18 years; where approved, adolescents (aged 12-17 years) weighing at least 30 kg may also be enrolled. A diagnosis of primary C3G or IC-MPGN (with or without previous renal transplant).  Evidence of active renal disease, based on one or more of the following:  In adults or adolescents with a baseline renal biopsy, at least 2+ staining for C3c on the baseline renal biopsy collected during screening, per the central pathology laboratory. In adolescents not providing a baseline renal biopsy, at least one of the following: Serum C5b-9 level above the upper limit of normal (ULN) during screening. Serum C3 below the LLN during screening. Presence of an active urine sediment during screening, as evidenced by hematuria with at least 5 red blood cells (RBCs) per high-power field (HPF) and/or red blood cell casts on local or central microscopic analysis of urine. Presence of C3 nephritic factor within 6 months of screening, based on central lab results or medical history. No more than 50% global glomerulosclerosis or interstitial fibrosis on the baseline biopsy, for adult subjects or adolescent subjects providing a baseline biopsy. At least 1 g/day of proteinuria on a screening 24-hour urine collection, and a uPCR of at least 1000 mg/g on at least 2 first-morning urine samples collected during screening. eGFR ≥30 mL/min/1.73 m2 calculated by the Chronic Kidney Disease-Epidemiology Collaboration creatinine equation for adults, or the Bedside Schwartz equation for adolescents.  Stable regimen for C3G/IC-MPGN treatment, as described below:  Stable and optimized angiotensin converting enzyme inhibitor (ACEi)/angiotensin receptor blocker (ARB) therapy for at least 12 weeks prior to randomization, in the opinion of the investigator. Stable doses of other medications that can affect proteinuria (eg, steroids, mycophenolate mofetil (MMF) and/or other allowed immunosuppressants that the patient is receiving for treatment of C3G) for at least 8 weeks prior to the baseline renal biopsy, and at least 12 weeks prior to randomization. Have received vaccinations against S pneumoniae, N meningitidis (types A, C, W, Y, and B), and H influenzae (type B) within 5 years prior to randomization or agree to receive vaccinations during screening.  Exclusion Criteria:  Previous exposure to pegcetacoplan. C3G/IC-MPGN secondary to another condition (eg, infection, malignancy, monoclonal gammopathy, a systemic autoimmune disease such as systemic lupus erythematosus, chronic antibody-mediated rejection, or a medication), in the opinion of the investigator. Current or prior diagnosis of human immunodeficiency virus (HIV), hepatitis B (HBV), or hepatitis C (HCV) infection or positive serology during screening that is indicative of infection with any of these viruses. Weight more than 100 kg at screening. Hypersensitivity to pegcetacoplan or to any of the excipients. History of meningococcal disease.  Malignancy, except for the following:  Cured basal or squamous cell skin cancer Curatively treated in situ disease Malignancy-free and off treatment for ≥5 years An absolute neutrophil count <1000 cells/mm3 at screening. Use of rituximab, belimumab, or any approved or investigational anticomplement therapy other than pegcetacoplan within 5 half-lives of that product prior to the screening period. Female subjects who are pregnant or who are currently breastfeeding and are unwilling to discontinue for the duration of the study and for at least 90 days after the final dose of study drug. Presence or suspicion of severe recurrent or chronic infections that, in the opinion of the investigator, may place the subject at unacceptable risk by study participation.