His first professional recording experience, at age 19, was performing acoustic guitar and background vocals on Bob Seger's single, "Ramblin' Gamblin' Man" in 1968. Frey has said that Seger strongly encouraged and influenced him to focus on writing original songs. They remained good friends and occasional songwriting partners in later years.
Frey then moved to Los Angeles to follow his then-girlfriend, Joan Sliwin, who was an aspiring singer. He was introduced to J. D. Souther by her sister, Alexandra Sliwin of Honey Ltd., a friend from his Detroit days and Souther's girlfriend at the time. Frey debuted as a recorded songwriter while fronting Longbranch Pennywhistle, a duo with Souther, in 1969. Frey wrote the songs "Run, Boy, Run" and "Rebecca" and co-wrote "Bring Back Funky Women" with Souther for the album Longbranch Pennywhistle. Frey also met Jackson Browne during this period. The three musicians lived in the same apartment building for a short time, and Frey later said that he learned a lot about songwriting from hearing Browne work on songs in the apartment below.