Morton Mower, an entrepreneurial cardiologist who helped invent an implantable defibrillator that has saved many lives by returning potentially fatal irregular heart rhythms to normal with an electrical jolt, died on April 25 in Denver. He was 89.

His son, Mark, said the cause was cancer.

Dr. Mower and Dr. Michel Mirowski, a colleague at Sinai Hospital in Baltimore, began work in 1969 on a device that would be small enough that it could be implanted under the skin of the abdomen and quickly correct a heart’s rhythms when they go dangerously awry.

Dr. Mirowski had the idea to miniaturize a defibrillator; Dr. Mower, who had taught himself electrical engineering in his basement workshop, believed it could be done.

“We were the crazy guys who wanted to put a time bomb in people’s chests,” Dr. Mower said in 2015 in an interview with the medical journal The Lancet, which noted at the time that two million people around the world had received the implantable device.

The doctors quickly developed a prototype and formed a partnership in 1972 with Medrad, a medical equipment maker. But the development of an implantable defibrillator had its critics.

Writing in Circulation, an American Heart Association journal, Dr. Bernard Lown, who invented the first effective external defibrillator, and Dr. Paul Axelrod said that patients with ventricular fibrillation were better served by surgery or an anti-arrhythmia program.

“In fact,” they said, “the implanted defibrillator system represents an imperfect solution in search of a plausible and practical application.”

The work continued. After being tested on animals, the battery-operated device, approximately the size of a deck of cards, was first implanted in humans at Johns Hopkins Hospital in 1980. Five years later, it was approved by the Food and Drug Administration.

At the time, the F.D.A. said the implantable defibrillator could save 10,000 to 20,000 lives a year by letting people have their arrhythmia corrected quickly rather than waiting to reach hospital emergency rooms, where external defibrillators, with their paddles, are used.

Dr. Donald M. Lloyd-Jones, president of the American Heart Association, said in a phone interview that 300,000 devices, now as small as a silver dollar, are implanted annually.

“Letting people walk around with a defibrillator, rather than being in a hospital under constant care, was really revolutionary in saving the lives of people at risk of fatal heart attacks,” Dr. Lloyd-Jones said.

He added that another advantage of the device — formally known as the automatic implantable cardioverter defibrillator — was that its electric shock is delivered directly to the heart. The external defibrillator’s jolt must travel from its paddles through skin and tissue before reaching the heart.

Dr. Mower and Dr. Mirowski were inducted into the Inventors Hall of Fame in 2002, along with Alois Langer, a project engineer at Medrad, and M. Stephen Heilman, the company’s founder.

Morton Maimon Mower was born on Jan. 31, 1933, in Baltimore and grew up in Frederick, about 50 miles west. His father, Robert, was a cobbler, and his mother, Pauline (Maimon) Mower, was a homemaker.

As a youngster, Morton worked during the summers for his Uncle Sam, who owned bathhouses and a toy store in Atlantic City. When his uncle got sick, Morton was impressed by how the family treated the doctor during his house calls.

“They made him sit down; they made him have a cup of tea,” Dr. Mower told the alumni magazine of the University of Maryland School of Medicine, from which he graduated in 1959, in an interview. “I thought, Gee, that’s not bad. That’s what I would like to do.”

After earning a bachelor’s degree from Johns Hopkins University in 1955, where he was in the pre-med program, and graduating from medical school, Dr. Mower completed an internship at the University of Maryland Medical Center.

He became chief resident at Sinai Hospital in 1962 and then served from 1963 to 1965 in the Army Medical Corps in Bremerhaven, Germany, where he was chief of medicine.

In 1966, he started a six-year stint as an investigator in Sinai’s coronary drug project. He eventually became an attending physician and chief of cardiology at the hospital. A building was named for him on its campus in 2005.

Dr. Mower became wealthy from licensing the defibrillator technology and used his money to build a large art collection that included works by Rembrandt, Picasso and Impressionist masters.

After leaving Sinai in 1989, he worked for two defibrillator makers: Cardiac Pacemakers, a subsidiary of Eli Lilly, as a vice president, and Guidant, as a consultant. He later taught medicine at Johns Hopkins and most recently, the University of Colorado school of medicine in Aurora.

Dr. Mower recently created a company, Rocky Mountain Biphasic, to find commercial uses for his many patents in areas including cardiology, wound healing, diabetes and Covid-19.

In addition to his son, he is survived by his wife, Toby (Kurland) Mower, a registered nurse; a daughter, Robin Mower; three grandsons; a brother, Bernard; and a sister, Susan Burke. He lived in Denver.

Dr. Mower’s work in resetting the heart’s rhythms didn’t end with the implantable defibrillator.

“I realized this was an incomplete therapy,” he told The Lancet, referring to the defibrillator. “It prevented right ventricular afibrillation, but it did nothing to support left ventricular function. People were stull dying of congestive heart failure.”

He and Dr. Mirowski went on to invent cardiac resynchronization therapy, or C.R.T., which uses an implantable device much like a pacemaker to send electrical impulses to the right and left ventricles of the heart in order to force them to contract in a more efficient, organized pattern.

“C.R.T. was every bit as big an advance as implantable defibrillators,” Dr. Mower said, adding that when he started testing the treatment on patients in the Netherlands, “It was almost unbelievable how the patients would come out of heart failure.”