Key Inclusion Criteria:  Diagnosis of Wilson Disease by Leipzig Score ≥ 4. Adequate venous access to allow collection of required blood samples. Able to swallow intact ALXN1840 tablets or mini-tablets. Willing to avoid intake of foods and drinks with high contents of copper. Willing and able to follow protocol-specified contraception requirements.  Key Exclusion Criteria:  Decompensated hepatic cirrhosis or MELD score > 13 (ages 12 to <18) or PELD score > 13 (ages 3 to < 12). Modified Nazer score > 7. Clinically significant gastrointestinal bleed within past 3 months. Alanine aminotransferase (ALT) > 2 × upper limit of normal (ULN) for participants treated for > 28 days with WD therapy or ALT > 5 × ULN for treatment-naïve participants or participants who have been treated for ≤ 28 days. Marked neurological disease requiring either nasogastric feeding tube or intensive inpatient medical care. Hemoglobin less than lower limit of the reference range for age and sex. History of seizure activity within 6 months prior to informed consent/assent. Participants in renal failure, defined as in end-stage renal disease on dialysis (chronic kidney disease stage 5) or estimated glomerular filtration rate < 30 milliliters/minute/1.73 meter squared.