William Warner completed his legal education at Lawrence University and the University of Michigan at Ann Arbor, and was admitted to the bar in 1861. He served in the Civil War, rising to the rank of major. Warner transitioned from military service to a legal career, holding various public offices in Kansas City, including city attorney, circuit attorney, and mayor. He also served in Congress and the Senate, where he chaired the Senate Committee on Mississippi River and Its Tributaries and focused on infrastructure and waterways. Warner's career reflects a blend of military service, legal expertise, and public administration.

William Warner's political identity is marked by an intriguing blend of support for regulatory measures and protective tariffs, suggesting a nuanced approach to economic policy. His advocacy for veterans' rights and pensions, alongside his legislative focus on improving river navigation and infrastructure, reveals a highly localized strategic philosophy. Warner's backing of the Pendleton Civil Service Reform Act hints at an unexpected commitment to meritocracy amidst his otherwise protectionist economic stances.