He worked in several advertising companies and ghostwrote the 1917 novel Mary Louise Solves a Mystery to satisfy his ailing father's publishing obligations.

Therefore, Harry ghostwrote the 1917 novel Mary Louise Solves a Mystery, which was part of The Bluebird Books series, mimicking his father's style.

[9] In 1944, Harry starred as President Woodrow Wilson in the play The Time to Come produced by the Little Theater of Western Springs.

[8] Baum also served as the host of a number of annual meetings held by The International Wizard of Oz Club.

[11] In 1942, he married Brenda Holter,[11] a pianist and composer who served as the president of Chicago's Musicians Club of Women.