Thomas was born in Washington, D.C.. He attended the American University for his undergraduate education. During the 1960s and early 1970s he worked as a reporter at NBC News. His column began in 1984 and appears in more than 550 newspapers throughout the United States. He is a panelist on Fox News Watch, a Fox News Channel program critiquing the media, and until September 2005 hosted After Hours with Cal Thomas on the same network. He also gives a daily radio commentary, which is heard on more than 300 stations.

Thomas has written extensively about political issues and he supports, among other things, many American positions related to Israel.

He has written 10 books, including Blinded By Might, that discussed, among other things, the role of the Moral Majority in American politics of the 1980s. Thomas was vice president of the Moral Majority from 1980 to 1985. He remains an evangelical Christian.

Support for removal of Palestinians from the Palestinian territories

In response to the Dolphinarium disco bombing in Tel-Aviv that killed 21 people, mostly high school students, Thomas authored a June 6, 2001, article in the Jewish World Review expressing support for expelling Palestinians to surrounding Arab countries.

Criticism of Dutch government policy

In 2004, Cal Thomas was the target of a Google bomb attack, where the phrase "ignorant asshole" was linked to his website. This Googlebomb was triggered by a segment on his After Hours show regarding the Netherlands' policy on euthanasia. It soon became an attack on the Netherlands' drug policy, prostitution, and the nation's immigration policy. In a follow-up, Thomas rejoined "Thanks to our Dutch viewers for watching and for caring enough to write. Keep watching. Maybe you'll see some things you do like."

Criticism of Islam

In his article "Mumbai Explained", published in the Chicago Tribune, Thomas writes that "no new [Mosques] should be built" in Western countries following the Mumbai terrorist attacks. He further claimed that Muslim immigration posed a danger to the UK and USA.

Criticism of homosexuality

After Bill Clinton became the first sitting United States president to address a gay advocacy organization, the Human Rights Campaign, Thomas published a column in November 1997 opposing homosexuality, in which he said:

Thomas published a similar column on October 16, 2009, after Barack Obama became the second sitting United States president to address the Human Rights Campaign. Thomas said:

Reporter at KPRC-TV

In the early 1970s Thomas was a reporter with KPRC-TV in Houston, Texas. He was an onsite reporter during Fred Carrasco's 1974 Huntsville Prison Siege, of which Thomas wrote, “It is a tragedy that two hostages died. It is a miracle all the rest lived.”