Best-selling author Stephen King testified Tuesday in his Justice Department lawsuit to block Penguin from buying Random House Simon & Schuster that a merger of the nation’s two biggest publishers would make life difficult for writers.

Mr. King testified as a witness for the government in a lawsuit filed in the U.S. District Court for the District of Columbia to stop the $2.18 billion acquisition.

The companies are the largest of the so-called Big Five publishers and are designed to consolidate part of an industry. Penguin Random House, which is owned by the German company Bertelsmann, is itself the result of a 2013 merger.

“Mergers are bad for competition,” Mr. King said of why he agreed to testify.

Penguin argues that the Random House acquisition will benefit both authors and readers. The lawyers say that under the deal, Simon & Schuster authors will have access to Penguin Random House’s supply chain and distribution networks, and that the savings from combining the two companies will result in higher payouts for authors.

Prosecutors used Mr. King’s testimony to bolster their argument that the consolidation of the publishing industry hurt authors and the industry.

Dressed in a gray suit and gray slip-on shoes, the testimony of the author of titles including “It” and “Pet Sematary” drew occasional laughs from the audience in the courtroom. Penguin Random House’s lawyers declined to cross-examine Mr. King after he spent half an hour testifying.

Mr. King said that when he started publishing in the mid-1970s, he had hundreds of prints in his business and bought his work without an agent. He said the number of publishers was dwindling as competing businesses went into hiding or collapsed.

With fewer imprints competing for business, advances gradually slowed, he continued, especially for writers with no sales records.

Not only that, the advance payments to writers based on expected royalties were especially generous to unknown writers. Mr. King said he received a $10,000 advance for his first books, including “Carrie” and “The Shining.”

“It’s getting harder and harder for writers to make enough money to live,” he said.

Over the following decades, Mr. King wrote several bestsellers and made a fortune from his books. That financial security, he says, often goes hand in hand with small independent publishers to publish books.

He said he accepted small advances from independent publishers because those imprints survived.

“If you’re very, very lucky, there will come a point where you can stop following your bank account and follow your heart,” he told me.