In April 1889, Jeff Reed (a Texan and relative of the Daggs family) was appointed to carry the mail from Stonewall to Center (which was later combined with Pickett), two small communities in then Indian Territory.

With his family and his stock, he sought a place for a home on a prairie midway between the two points, where he constructed a log house and started Reed's Store.

[6] Ada incorporated as a city in 1901 and grew rapidly with the arrival of the St. Louis and San Francisco Railway line.

In the 1900s, the town was opened up to African Americans so that black witnesses could stay while testifying in district court.

"[8] When the threats went unanswered, unnamed parties blew up a Black restaurant with dynamite, seriously injuring one occupant.

"[8] After the incident, the town remained open to African Americans to provide labor for a local cotton compress.

[7] On April 19, 1909, an organized mob hanged four men, among whom was American outlaw Deacon Jim Miller, who was set to be tried for the murder of a former U.S. marshal and member of the local freemason lodge.

[11] The Daily Ardmoreite reported that the four lynched men were "one of the bloodiest band of murderers in the state of Oklahoma and an organization of professional assassins, that for a record of blood crimes, probably has no equal in the annals of criminal history in the entire southwest".

[7] LegalShield, a multi-level marketing provider of pre-paid legal services, is headquartered in the city.

The largest employers in the region are:[25] East Central University, located in Ada, is a public four-year institution that has been in operation since 1909.

[29][30] Because of its short, palindromic spelling with frequently used letters, Ada is a very common crossword puzzle answer.

"[51] In 1987, journalist Robert Mayer published The Dreams of Ada exploring major flaws, irregularities, forced confessions, and possible miscarriages of justice in Ada in the convictions of Tommy Ward and Karl Fontenot for the rape and murder of Denice Haraway, who died in 1984.

In 2006, John Grisham brought Ada into the national spotlight in his nonfiction work The Innocent Man, relating a similar story in the convictions of Ron Williamson and Dennis Fritz for the murder of Debra Sue Carter.

After 12 years on death row, DNA evidence proved the men's innocence and established the guilt of the prosecution's main witness.