Thornton started his career as a "singing waiter" in Boston, Massachusetts, United States, and then achieved success with his wife, Elisabeth "Bonnie" Cox, in music halls throughout the US as what was then called a "serio-comic" or "monologist" (essentially a stand-up comic) and singer.

[1] Thornton's compositions included: "When You Were Sweet Sixteen",[1] "She May Have Seen Better Days", "The Irish Jubilee", "Two Little Girls in Blue", "When Summer Comes Around", "It Don't Seem Like the Same Old Smile", "My Sweetheart's the Man in the Moon",[1] "Going for a Pardon",[2] and "The Streets of Cairo".

[5] According to the 1900 US census and his birth certificate,[citation needed] he was born of Irish parents, John Thornton and Catherine Molloy, in Liverpool, England.

Thornton, returning home from a drinking spree on Independence Day, was met with Bonnie's tearful reproof and her sadly expressed doubt of his love and fidelity.

The words gave him the title and idea for one of his greatest songs, which was later sung with enormous success by Bonnie at Tony Pastor's 14th Street Theatre.Apparently, this was a recurring theme in their marriage, as a similar story is told about the origin of "When You Were Sweet Sixteen".