ST. LOUIS — When Sister Mary Jean Ryan entered the convent as a young nurse in 1960, virtually every department of every Catholic hospital was run by a nun, from pediatrics to dietary to billing. After her retirement July 31 as the chief executive of one of the country’s largest networks of Catholic hospitals, only 11 nuns remained among her company’s more than 22,000 employees, and none were administrators.

For SSM Health Care, a $4.2 billion enterprise that evolved from the work of five German nuns who arrived here in 1872, Ryan’s departure after 25 years as the company’s first chief executive marks a poignant passing. The gradual transition from religious to lay leadership, which has been changing the face of Catholic health care for decades, is now nearly complete.

In 1968, nuns or priests served as chief executives of 770 of the country’s 796 Catholic hospitals, according to the Catholic Health Association. In 1969, St. Charles Bend came under the leadership of Sister Catherine Hellmann, who would serve as president and CEO for more than 25 years.

Today, nuns preside over 8 of 636 hospitals. With Ryan’s departure, only 8 of 59 Catholic health care systems are directed by religious executives.

SSM, which is now led by William Thompson, a Catholic layman and longtime company executive, had been the largest Catholic health system still managed by a nun. Formed in 1986, the St. Louis firm consolidated the management of 15 Catholic hospitals and two nursing homes in Missouri, Illinois, Oklahoma and Wisconsin.

As with other healing orders that have ceded control, the Franciscan Sisters of Mary prepared for their inevitable detachment from SSM with more planning than sentiment.

“We can’t be maudlin about this,” said Ryan, 73, who still presides over the company’s board. “I mean, yes, we are a dying breed. We are disappearing from the face of the earth and all of that. That being said, perhaps this is a moment for people to acknowledge the contribution that has been made by women religious throughout our history in the United States.”

The near extinction of nuns from U.S. hospitals stems largely from the drastic decline of religious orders that accompanied the women’s movement, the sexual revolution, ethnic assimilation and the Second Vatican Council’s opening of the church to lay leadership.

Even as the country’s Roman Catholic population surged by nearly 50 percent over the last half-century, the number of nuns dropped precipitously, to 56,000 today from 180,000 in 1965, according to the Center for Applied Research in the Apostolate at Georgetown University. In 2009, 91 percent of all nuns were at least 60 years old.

Along with parochial education, health care has long been a central mission of nuns in this country, a natural outgrowth of the Catholic insistence on the sacredness of life. Since 1727, when the Ursuline sisters landed in New Orleans, they have built 12 of the country’s 40 largest health care systems. In 2009, Catholic hospitals accounted for one of every six admissions in the U.S., according to the Catholic Health Association.

Other than crucifixes on the walls and marble Madonnas in the lobby, Catholic hospitals do not look particularly different from secular ones. But their administrators say that what makes them distinct is a values-driven approach, reflected at SSM in a mission statement that pledges to use exceptional care to “reveal the healing presence of God.”

Catholic health systems have been criticized, along with other nonprofit hospitals, as not dedicating enough resources to the community’s benefit. But surveys also show that, on average, they provide higher quality performance than other hospitals and are more likely to offer specialty services that are not profit centers.

Ryan, who was dispatched by her order to earn a master’s degree in hospital administration, has managed her realm with iron-willed competence and unblinking compassion, SSM executives said.

Operating revenues at SSM (the initials honor the Sisters of St. Mary, a predecessor order) more than quintupled during Ryan’s tenure, to $3 billion in 2010. The company produced net income that year of $247.9 million and provided $115.4 million in uncompensated care.

A decade ago, Ryan led SSM through a relentless campaign to improve performance that was recognized with the first Malcolm Baldrige National Quality Award given to a health care company.

But her legacy also extends to preaching about the dignity of patients, paying blue-collar workers above scale, making her hospitals smoke-free, banning the use of foam cups and plastic water bottles, and insisting on gender-neutral and nonviolent language.