The company is based in St. Louis, with 2.4 million electric, and 900,000 natural gas customers across 64,000 square miles in central and eastern Missouri and the southern four-fifths of Illinois by area.

[6] Prior to the formation of Ameren, the first major development in the history of its constituent parts occurred in 1929, when the Bagnell Dam was completed on the Osage River and generated almost 175 megawatts of hydroelectricity for Missouri's Union Electric Company.

Union Electric later bought the dam, providing 134 megawatts of hydroelectricity carried over a longer distance than had ever been achieved before.

[11] In 2003, Ameren acquired Peoria-based CILCORP, Inc. and its leading subsidiary, Central Illinois Light Company, from AES Corporation.

[12] CILCORP had traded on the NYSE with ticker symbol CER prior to its acquisition by AES, and by the mid-1990s had become a member of the S&P Small Cap 600 index.

[15] In 1991, Illinois Power reorganized as a holding company, Illinova Corporation, which traded on the NYSE with ticker symbol ILN.

[8] In a merger completed February 1, 2000, Illinova became a wholly owned subsidiary of Dynegy Inc. (NYSE: DYN), in which Chevron Corporation also took a 28% stake.

In 2015, Ameren became the first major energy company to open an Innovation Center at the Research Park, University of Illinois at Urbana-Champaign.

[22][23] As of 2017 Ameren shares are mainly held by institutional investors (Vanguard group, BlackRock, State Street Corporation, among others[24]).

Ameren Missouri owns the Taum Sauk pumped storage plant,[25] which failed on December 14, 2005, causing extensive damage to the east fork of the Black River and to Johnson's Shut-Ins State Park.

EPA alleged Ameren violated the Clean Air Act, the Missouri State Implementation Plan, and Ameren's Rush Island Plant Title V Permit when it undertook major modifications at the Rush Island Plant in 2007 through 2010 without obtaining the required permits under the Prevention of Significant Deterioration (PSD) provisions of the New Source Review (NSR) program.

In January 2017 the District Court found that Ameren "...violated the Clean Air Act without obtaining the necessary permits, installing best-available pollution technology, and otherwise meeting applicable requirements."

Judge Rodney Sippel entered a finding of liability in January 2017 and ordered a remedy that called for the company to equip its Rush Island coal plant in Jefferson County, Missouri, with scrubbers.

In 2016, the company attempted to launch a pilot program to install electric vehicle charging stations but state utility regulators turned down oversight of the project.

[29] In nineteenth and twentieth centuries a local utility company operated a coal gasification plant at the corner of Ash and Orr streets in Columbia, Missouri, that was subsequently demolished.

[31][32] Ameren has been criticized by the North Village Board of Directors for owning "a whole city block that is in disrepair and for an aversion to commerce.

"[33] Ameren Missouri sponsors the Adopt-the-Shoreline program, which enables people to “adopt” portions of the shoreline for litter control.

[35] In 2023, a Sierra Club report included Ameren's Labadie Energy Center among the 17 deadliest coal plants in the US, with calls to the EPA to pass regulations on harmful emissions contributing to regional haze affecting neighboring communities.

[37] Labadie received notice of Clean Air Act violations from the EPA in at least 2010 and 2011, being cited for “major modifications that caused a significant net emissions increase” without first obtaining proper permits.