Thomas starred in an ABC pilot called Two's Company in 1965. Although it did not sell, it caught the attention of an ABC programming executive. He met with Thomas, and expressed interest in casting her in her own series. With their encouragement, Thomas came up with her own idea for a show about a young woman who leaves home, moves to New York City, and struggles to become an actress. The network was initially hesitant, fearing audiences would find a series centering on a single female uninteresting or unrealistic. Thomas, however, ensured the show's success and it was put into production. 　　The concept eventually evolved into the sitcom entitled That Girl, in which Thomas played Ann Marie, a beautiful, up-and-coming actress with a writer boyfriend, played by Ted Bessell. The series told the daily struggles of Ann holding different temporary jobs while pursuing her dream of a career on Broadway. The series was the first television show to focus on a working, single girl who did not live with her parents, and paved the way for many shows to come. Thomas was only the second woman, following Lucille Ball, to produce her own series. That Girl aired from 1966 to 1971, producing 136 episodes, and was a solid performer in the Nielsen ratings. 　　In 1971, Thomas chose to end the series after five years. Both ABC and the show's sponsor, Clairol, wanted the series finale to be a wedding between the two central characters, but Thomas rebuffed them, saying that she felt it was the wrong message to send to her female audience, because it would give the impression that the only happy ending is a marriage. That Girl has since become popular in syndication.