Answer by taking a quote from the following article:

Kasem was born in Detroit, Michigan, on April 27, 1932, to Lebanese Druze immigrant parents, who had settled in Michigan, where they worked as grocers. Kasem was named after Turkish leader Mustafa Kemal Ataturk, a man Kasem said his father respected. In the 1940s, "Make Believe Ballroom" reportedly inspired Kasem to follow a career in radio and later host a national radio hits countdown show. Kasem received his first experience in radio covering sports at Northwestern High School in Detroit.

On July 4, 1970, Kasem, along with Don Bustany, Tom Rounds, and Ron Jacobs, launched the weekly radio program American Top 40 (AT40). At the time, top 40 radio was on the decline as DJs preferred to play album-oriented progressive rock. Loosely based on the TV program Your Hit Parade, the show counted down from #40 on the pop charts to #1--the first #1 was Three Dog Night's "Mama Told Me (Not to Come)"--based on the Billboard Hot 100 each week. The show, however, was not just about the countdown. Kasem mixed in biographical information about the artists, flashback, and "long-distance dedication" segments where he read letters written by listeners to dedicate songs of their choice to far away loved ones. He often included trivia facts about songs he played and artists whose work he showcased. Frequently, he mentioned a trivia fact about an unnamed singer before a commercial break, then provided the name of the singer after returning from the break. Kasem ended the program with his signature sign-off, "Keep your feet on the ground and keep reaching for the stars."  The show debuted on seven stations, but on the back of Kasem's "always friendly and upbeat" baritone voice it soon went nationwide. In October 1978, the show expanded from three hours a week to four. American Top 40's success spawned several imitators including a weekly half-hour music video television show, America's Top 10, hosted by Kasem himself. "When we first went on the air, I thought we would be around for at least 20 years," he later remarked. "I knew the formula worked. I knew people tuned in to find out what the No. 1 record was." Due to his great knowledge of music, Kasem became known as not just a disc jockey, but also a music historian.  In 1971, Kasem provided the character voice of Peter Cottontail in the Rankin/Bass production of Here Comes Peter Cottontail. In the same year, he appeared in the low-budget film The Incredible 2-Headed Transplant, in what was probably his best remembered acting role. From 1973 until 1985, he voiced Robin on several SuperFriends franchise shows. In 1980, he voiced Merry in The Return of the King. He also voiced Alexander Cabot III on Josie and the Pussycats and Josie and the Pussycats in Outer Space, and supplied a number of voices for Sesame Street.  In the late 1970s, Kasem portrayed an actor who imitated Columbo in the Hardy Boys/Nancy Drew Mysteries two-part episode "The Mystery of the Hollywood Phantom." He portrayed a golf commentator in an episode of Charlie's Angels titled "Winning is for Losers", and appeared on Police Story, Quincy, M.E., and Switch. In 1977 he was initially hired as the narrator for the ABC sitcom Soap, but quit after the pilot episode due to the content. Rod Roddy took his place on the program. In 1984, Kasem made a cameo in Ghostbusters, reprising his role as the host of American Top 40. For a period in the late 1970s, Kasem was also the staff announcer for the NBC television network.

How long did the show run?
In October 1978, the show expanded from three hours a week to four. American Top 40's success spawned several imitators including a weekly half-hour music