WASHINGTON — Saint John Barrett was often away from home when his five children were young. He did not tell them where he was going or say much about the work he did.

It took years before they learned that, during the height of the civil rights movement, their father traveled across the South, helping to define a new branch of law and attempting to bring an end to segregation.

Beginning in 1955, when he came to Washington, Mr. Barrett was one of the first civil rights lawyers in the government. He was part of the Justice Department’s Civil Rights Division when it was created in 1957 and had a major role in many celebrated legal landmarks, including the desegregation of Little Rock’s Central High School in the 1950s, James Meredith’s enrollment as the first black student at the University of Mississippi, and to the integration of interstate buses by the Freedom Riders of the early 1960s.

Mr. Barrett, who was 89 when he died May 28 at Howard County (Md.) General Hospital of pneumonia, seldom made headlines. But for more than a decade, he was at the forefront of perhaps the most momentous movement for social change in the nation’s history.

‘‘He made an enormous impact as a government lawyer in enforcing the civil rights laws,’’ said John Doar, the top lawyer in the Civil Rights Division in the 1960s.

In 1957, Mr. Barrett worked with Thurgood Marshall — who later became the first black justice on the US Supreme Court — on the case in Little Rock in which the governor used the National Guard to prevent the school from integrating.

Often, however, Mr. Barrett was on his own, exploring a new legal field with few precedents. Armed with little more than the force of law and sheer moral courage, he performed much of his work in the face of intimidation, anger and fear.

At home in Chevy Chase, Md., Mr. Barrett’s children knew little about their father’s contributions to civil rights.

‘‘He didn’t talk about that,’’ said his son David, of Bethesda, Md. ‘‘That’s something he would downplay.’’

Saint John Barrett was born in Santa Rosa, Calif., where his father was a lawyer. The younger Barrett — whose first name derived from his mother’s maiden name — grew to be a lanky 6-foot-4 and was known from an early age as Slim.

He graduated from Pomona College in Claremont, Calif., in 1943. He graduated from law school at the University of California at Berkeley in 1948.

At the Justice Department, Mr. Barrett handled voting rights and school desegregation cases — including an infamous episode in Virginia, when Prince Edward County officials closed the public schools for five years rather than comply with an order to desegregate.

Although he said he never felt in personal danger, Mr. Barrett was a firsthand witness to how the racial order of the South was enforced by violence.

In 1962, he accompanied Meredith as he tried to enroll at the all-white University of Mississippi. State troopers formed a cordon through which Meredith and Mr. Barrett had to pass.

When Governor Ross Barnett, a Democrat, refused to admit Meredith to the university, Mr. Barrett told the governor that he was violating a federal court order. As they left the campus in a car, Mr. Barrett was seated next to Meredith in the back seat.

‘‘A pretty coed stood a couple of feet from my closed car window,’’ Mr. Barrett recalled in a 2009 memoir. ‘‘She was looking directly at me shouting something I could not distinguish, her face contorted with rage as she shook her middle finger.’’

In another case, Mr. Barrett brought charges against Lester Maddox, a Georgia restaurant owner who became governor, for refusing to serve black customers.

‘‘Dad felt zero fear going into those situations in the South,’’ his son James, of Garrett Park, Md., said of his father. ‘‘It just didn’t bother him.’’

Mr. Barrett left the Justice Department in 1967 to become deputy general counsel at the old Department of Health, Education and Welfare. He went into private practice in 1977 and retired in 2002.

Besides his sons James and David, Mr. Barrett leaves his wife of 52 years, Elisabeth Fuchs Barrett of Ellicott City; two daughters, Susan Borchers of Ellicott City, and Anna Hodgson of Washington; and another son, and Robert of San Francisco, a sister; and 13 grandchildren