Aaron Beck liked to tell a story about how he came to reject psychoanalysis and revolutionize how talk therapy for mental disorders was conducted in the United States and much of the world.

Like other psychiatrists in the mid-20th century, Dr. Beck was trained in Freudian concepts, including the idea that depression was the result of anger turned inward. In what would become a lifelong pattern, he decided in the late 1950s to test that idea more scientifically. He found little evidence that his patients were angry inside, but they did suffer from negative, irrational thinking about themselves.

He told of a psychoanalysis patient who was worried that her stories about her sexual experiences were boring him. They definitely were not. He began asking other patients what they were thinking and found that they, too, were feeding themselves a diet of negative thoughts. They saw themselves as failures in love and life.

Dr. Beck, who died Monday at his Rittenhouse Square condominium at the age of 100, wondered if it wouldn’t be better for patients to learn to think more logically and accurately in the present than to spend countless billable hours analyzing childhood slights. Cognitive therapy — now known as cognitive behavior therapy (CBT) — was born.

Dr. Beck, who spent nearly seven decades working and teaching at the University of Pennsylvania, as well as psychologist Albert Ellis, who explored similar ideas independently, are credited with developing a faster, evidence-based treatment for depression and, later, many other mental health maladies. With more than 2,000 studies of its impact, CBT is the most studied non-drug mental health treatment. Research has generally found CBT to be at least as effective as medications for non-psychotic disorders and to have longer-lasting results, said Steven Hollon, a Vanderbilt University psychologist who trained with Dr. Beck.

“He’s the single most important psychiatrist in the world of the 20th century,” said Martin Seligman, a Penn psychologist who is famous in his own right as the founder of positive psychology. He and Dr. Beck met once a month for the last decade to talk shop and share books.

Robert DeRubeis, a psychologist who heads Penn’s clinical training program for psychologists, said he would amend Seligman’s evaluation. “I would put him as arguably the most important figure in mental health in the last century,” he said. “I wouldn’t narrow it to psychiatry.”

“He was a giant,” said Frank Farley, emeritus professor of psychological studies in education at Temple University and former president of the American Psychological Association (APA). He said that Dr. Beck, along with Ellis, dethroned Sigmund Freud, the highly influential creator of psychoanalysis. Farley thought Dr. Beck deserved a Nobel Prize.

Seligman said that Dr. Beck was, in fact, short-listed for that award but did not win it. (A 2008 Inquirer story said Penn had booked a limo in 2007 just in case Dr. Beck won.) He did, however, win the Heinz Award for the Human Condition in 2001 and the Albert Lasker Award for Clinical Medical Research in 2006. He also wrote or cowrote 25 books.

Arthur C. Evans Jr., executive director of the APA and former commissioner of Philadelphia’s Department of Behavioral Health and Intellectual disAbility, lauded Dr. Beck for bringing “hope and healing” to people with mental health and substance use disorders. “He was truly one of the most extraordinary human beings I have ever known,” Evans said. “His legacy will be profound and enduring.”

Aaron Temkin Beck, who was Tim to his best friends and ATB to coworkers at the Beck Institute for Cognitive Behavior Therapy, was a warm, deeply curious, compassionate, self-effacing man who never quit working.

Two days before his death, Dr. Beck spent hours on the phone with Paul Grant, director of the institute’s Center for Recovery-Oriented Cognitive Therapy, discussing a paper they were writing together that they called “Keys to Their Kingdom.” It explained theories they’d been developing over the last two decades to help patients with serious mental illnesses, including schizophrenia, benefit from CBT. Working with this often-neglected group had become Dr. Beck’s passion. The two were focused on tapping into periods of “adaptive mode” when patients were receptive to communication and change.

Asked why Dr. Beck continued to work so late into his life, Grant said his mentor, who was already at work before Grant was born, was always excited about new ideas. But he suspected that some of his late-life enthusiasm stemmed from the fact that his ideas were rejected by many of his peers until he was over 50. Few came to his early talks. Psychoanalysts dismissed him. “I always thought … he was making up for lost time,” Grant said. After a fall that broke a hip in January, Grant said, Dr. Beck’s desire to work became more urgent. “He was worried this was going to happen, that he wasn’t going to get a chance to see the paper finished.”

Friends said Dr. Beck, who was the son of Russian immigrants who settled in Providence, R.I., always maintained a healthy balance between work and family life. He married Phyllis Whitman in 1950. She served as vice dean of the University of Pennsylvania Law School and was the first woman to serve on Pennsylvania’s Superior Court. They had four children. He cofounded the Beck Institute in 1994 with his daughter Judith Beck, who is a psychologist.

He was an avid reader who favored histories. Although macular degeneration had taken his eyesight, he liked listening to books. Seligman suggested Our Woman in Moscow, a Cold War spy novel, less than two weeks before Dr. Beck died. Before his vision grew too poor to see the ball in his late 80s, Dr. Beck also played tennis frequently. DeRubeis, who is 32 years younger, said Dr. Beck wanted his help in improving his play at the net when he was around 70.

Dr. Beck himself loved telling a story about watching Boris Becker, famous for his meltdowns during games, play at Wimbledon, DeRubeis said. Becker, who was frustrated by his performance that day, turned to the crowd and asked, “Does anyone want to play for me?”

Dr. Beck, who was seated near the front and was probably in his 60s then, stood up and said, “I will.”

“Sit down!” the crowd yelled. Dr. Beck got a kick out of it.

Prominent psychologists said Dr. Beck’s greatest contribution was in questioning the prevailing treatment of his day — psychoanalysis — and giving patients a way to harness their own mental powers for positive change. Seligman said that Dr. Beck’s predecessors believed that the thoughts of mentally ill people were caused by roiling, conflicted emotions beneath the surface. “Beck,” he said, “turned it on its head and said, ‘No, it’s our thinking that changes our emotion.’”

Hollon said Dr. Beck’s key insight was that his patients might actually believe what they were telling him.

He would challenge patients to reexamine their thoughts and change their habits. It was a practical approach that often made a difference quickly. Grant said Dr. Beck joked that he knew the technique worked because his practice cleared out. “People actually got better,” he said.

Dr. Beck, and therapists who learned his techniques, believed in scientific testing, and they soon began building evidence that the treatment worked. Hollon said psychoanalysts have never embraced that kind of testing, so there are no good head-to-head comparisons.

CBT is now used for a wide variety of mental illnesses, including depression, eating and personality disorders, anxiety, and addiction. Dr. Beck also developed a widely used measure of depression symptoms.

He was generous with his time and attention when it came to students and was supportive of young researchers. DeRubeis said he met weekly with Dr. Beck for 20 years. He would sometimes bring students with him. Dr. Beck invariably asked each one what he or she was working on.

In July, a virtual gathering for Dr. Beck’s 100th birthday attracted about a dozen good friends, many of whom were former trainees. A running theme in their remarks, Hollon said, was that Dr. Beck had noticed them early and helped them. “This was a guy who spotted young talent and brought it along,” he said.

That support for other clinicians helped spread CBT and the research that showed it worked, said Lisa Pote, executive director of the Beck Institute. “He planted seeds through the people he worked with, and they planted seeds,” she said.

Dr. Beck is survived by his wife and four children, Roy, Judith, Daniel, and Alice Beck Dubow, as well as 10 grandchildren and 10 great-grandchildren. The family service will be private. The Beck Institute will host a public memorial at a future date.