The original AT40 program concept was created by Ron Jacobs, who with Tom Rounds founded Watermark in 1969. The program grew out of collaboration between Jacobs and K-B Productions owners Casey Kasem and Don Bustany, who sold Jacobs on the idea of a national music countdown. Despite the initial downturn in the Top 40 music format as the rapid proliferation of new FM stations popularized album-oriented rock in the early 1970s, AT40 soon found a loyal audience.

     AT40 was the first program to turn the popular local Top 40 countdown into a national syndicated show. The three-hour show was distributed weekly on records to radio stations across the United States, using the Billboard Top 100 as the source for the countdown. AT40 was distributed as a boxed record set each week. Records played 30 minutes of the show per side, and the set contained cue sheets allowing stations to integrate local station breaks into the AT40 program format. The records had to be played in the right order for the countdown to progress correctly. Program segments opened or ended with jingles identifying the program and the program host. By 1978 the general length of popular songs had increased, causing AT40 to increase its program length from three to four hours per show. Today the show is distributed on compact discs.

     One of the reasons for AT4o's success was the charismatic, personal voice style of Casey Kasem, the show's longest-serving host. The format initially called for a fast-paced delivery with minimal talk and a quick turnover from song to song. As the program gained momentum, Kasem's knowledge of popular music and his ability to create a sense of intimacy added interest for listeners. As the show expanded its time and found a loyal audience, special features such as the "Long Distance Dedication" became popular segments. Kasem's classic sign­ off, "Keep your feet on the ground and keep reaching for the stars" became the show's trademark.

     AT4o's success was challenged in 1979 with the introduction of The Weekly Top 30 hosted by Mark Elliot and in 1980 with Dick Clark's National Music Survey. Both of these shows were aimed at slightly different demographics than AT40. The Weekly Top 30 ended in 1982, and in 1983, Rick Dees' Weekly Top 40 aired based on the popular music chart listings in Cashbox magazine.

     In 1988 American Broadcasting Companies (ABC) Radio Networks, ·which had acquired Watermark, and Kasem were unable to agree to terms for a renewal contract. By this time AT40 had grown to become the most successful American radio program and was the sixth largest syndicated broadcast­ program with an estimated 2.4 million listeners worldwide. The show boasted nearly 1,000 outlets around the world. In July 1988 ABC introduced Shadoe Stevens as the new host of AT40. ABC heavily promoted the transition and introduced various new features to distinguish the new show host and keep the format fresh. Stevens hosted the show until 199 5. Several broadcasting companies vied for Kasem's talents, and in 1989 he signed a multimillion-dollar, multiyear contract with Westwood One to start a competing program called "Casey's Top 40."

     The early 1990s saw a substantial change in popular music. In November 1991 Billboard changed the way it tabulated the Hot 100. Billboard's new methodology led to a substantial increase in rap and other nontraditional pop music genres in the chart, causing many older loyal listeners to tune out. The traditional Top 40 format splintered into derivative formats. In addition, the continuing success of Rick Dees's Weekly Top 40 and Casey's Top 40 splintered the market for Top 40. By 1992 AT40 had fewer than 275 stations carrying the program in the United States, although it still held the predominant position among overseas listeners.

     In 1994 ABC Radio Networks acquired the Westwood One network and ownership of AT40. ABC now owned both the Rick Dees countdown and AT40. On 24 June 1994, ABC announced that it would cancel the American version of AT40, and the last program aired in January 1995. ABC's rights to the program terminated in 1998, and the show reverted back to Kasem and Bustany, the owners of K-B Productions. AT40 was revived with Kasem as the host in March 1998 under the ownership of AMFM Networks. In addition to the Top 40 format, there is also an American Top 20 based on the hot adult contemporary format and another geared toward adult contemporary listeners. Even though the format of the new AT40 is very similar to the original, the chart list is now based on the Mediabase 24/7 hit music charts.

     In 2003, the distribution of the program changed to Premiere Radio Networks. American Top 40 was at that time heard on 127 U.S. stations and 14 outlets internationally.