According to the United States Census Bureau, the city has a total area of 8.55 square miles (22.14 km2), all land.

The Headquarters Branch of the St. Louis County Library is located in Ladue on Lindbergh Boulevard (US 67).

Original Township 45 farming families included the Dennys, Dwyers, Conways, McCutcheons, McKnights (all Irish), Litzsinger, von Schraders, Spoedes, Luedloffs, Muellers, Seigers (all German), LaDues (French), Warsons, Lays, Barnes, Prices, and Watsons (all English), according to a 1868 Pitzman map of St. Louis,[16] as well as 1878 and 1909 maps of St. Louis County.

Once automobiles replaced horse and wagon as the primary mode of transportation, farmers in the area began selling portions of their land to city workers who wished to live outside of the urban setting.

Peter Albert LaDue was born in Kinderhook, New York, in 1821, a descendant of Pierre LaDoux, who arrived from France in the 1600s.

He arrived in Saint Louis about 1848 and later became a prominent attorney, alderman, and banker and land speculator.

The ensuing legal battle went to the United States Supreme Court which unanimously ruled, in City of Ladue v. Gilleo, that the right to place the sign was protected by the Constitution.

[18] In 1986, the City of Ladue sued residents E. Terrence Jones and Joan Kelly Horn for living together without being married.

The Missouri Court of Appeals sided with the city, stating in City of Ladue v. Horn that "A man and woman living together, sharing pleasures and certain responsibilities, does not per se constitute a family in even the conceptual sense.

[...] There is no doubt that there is a governmental interest in marriage and in preserving the integrity of the biological or legal family.

There is no concomitant governmental interest in keeping together a group of unrelated persons, no matter how closely they simulate a family.

Further, there is no state policy which commands that groups of people may live under the same roof in any section of a municipality they choose.

"[20] Under Chapter 213 of the Missouri Human Rights Act (§213.040.1),[21] passed after the Ladue v. Horn case, housing discrimination on the basis of familial status is now illegal.

In 2010, the former chief of police, Larry White, sued the City of Ladue for wrongful termination.

The resulting "disparity index" indicates a black driver was 15.98 times more likely than the average driver to be stopped by the Ladue Police Department in 2014, but the police department contends the statistics are skewed by the local racial composition.