Acthar Gel (repository corticotropin injection) is a naturally sourced complex mixture of adrenocorticotropic hormone analogs and other pituitary peptides that is believed to have both steroidogenic and nonsteroidogenic immunomodulatory effects via activation of melanocortin receptors in various cells throughout the body. Since 1952, Acthar has been approved by the US Food and Drug Administration to treat a variety of autoimmune and inflammatory diseases. Since 2014, Mallinckrodt Pharmaceuticals has conducted a large number of preclinical, clinical, and real-world-evidence studies of Acthar for the treatment of rheumatoid arthritis, systemic lupus erythematosus, dermatomyositis and polymyositis, multiple sclerosis relapse, ophthalmic disorders, sarcoidosis, and nephrotic syndrome. To date, Acthar has been the subject of more than 500 publications, many of which demonstrate the safety and efficacy of Acthar in patients with inflammatory diseases for whom standard treatments were ineffective or intolerable. Here, we review the history of Acthar and the findings of studies that have investigated the mechanism of action, safety, efficacy, and real-world effectiveness of Acthar for the treatment of inflammatory diseases.