He moved to New York City in 1914, as an arranger for the publishing firm, and began composing music in association with Pete Wendling.

He then started working with lyricist Jack Yellen, and they wrote together for the 1920 Broadway show What's in a Name, featuring the song "A Young Man's Fancy".

He composed many hit songs over the next decade, many with lyrics by Yellen, including “The Last of the Red Hot Mamas!”, "Lovin' Sam (The Sheik of Alabam')” (1922), “Hard Hearted Hannah (The Vamp of Savannah)” (1924), "Ain't She Sweet” (1927), and “Happy Days Are Here Again” (1929).

He died in Inglewood, California, in the same year, aged 85,[5] and was interred in the Westwood Village Memorial Park Cemetery in Los Angeles.

In 2007, a revue of Ager's music called Vampin' Lady opened in New Hope, Pennsylvania, performed by singer Joyce Moody under the direction of Earl Wentz and transferred to New York City as part of the American Composer Series.