Cashill was born and raised in Newark, New Jersey to William and Frances Cashill. He graduated from Regis High School in New York City and Siena College in Loudonville, NY. Cashill received his Ph.D. in American Studies from Purdue University in 1982.

Cashill has written at least seven books including Ron Brown's Body: How One Man's Death Saved the Clinton Presidency and Hillary's Future; 2006: The Chautauqua Rising; Hoodwinked: How Intellectual Hucksters Have Hijacked American Culture; Sucker Punch: The Left Hook that Dazed Ali and Killed King's Dream; and, with James Sanders, First Strike: TWA Flight 800 and the Attack on America. His newest book, What's The Matter With California? a reference to Thomas Frank's What's the Matter with Kansas? was released on October 2, 2007. He has also written and produced several documentaries, such as Silenced: TWA Flight 800, Mega Fix and the The Royal Years.

He has written for Fortune, The Washington Post, The Wall Street Journal, and The Weekly Standard.

Cashill hosted a daily talk radio show for KMBZ for four years. He has taught media and literature at Purdue and at other universities in the Kansas City area. He served as a Fulbright professor in Nancy, France.