Obviously, Carole King is the ultimate example of a songwriter who transitioned from doing behind-the-scenes work for other artists to becoming a star in her own right. And though she was reticent to perform or tour until after the success of Tapestry, she actually recorded quite a bit as an artist in the decade before that when she and Gary Goffin were enormously successful Brill Building songwriters. She released several solo singles in the '50s and '60s, some on Don Kirshner's Dimension Records label. When Dimension landed a huge hit with "The Loco-Motion," a song King and Goffin wrote for their babysitter Little Eva, the label released The Dimension Dolls, a compilation album featuring several songs apiece by King, Little Eva, and The Cookies (a later lineup of the '50s girl group whose original members had become Ray Charles' Raelettes). The five Carole King songs on that album included her first Top 40 hit as an artist, "It Might As Well Rain Until September," but it would be 9 years before she had another. 

After King divorced Goffin and moved to California, she formed a band with future husband Charles Larkey and Danny Kortchmar, a brilliant guitarist and songwriter perhaps best known for his work with Jackson Browne who'd play a major role in many of King's solo albums. That band, The City, released one album in 1968, and it's a pretty underrated little gem. Two songs from that album later appeared on King's 1980 solo album Pearls. To me it's kind of a mystery why Carole King's first solo album Writer basically did nothing commercially, even though it was released 7 months before Tapestry on the same label, with largely the same sound, and a single featuring James Taylor, whose career had just exploded that year. Writer isn't as good an album as Tapestry, sure, but not by a huge margin, in my opinion.