With help from her brother, Christian, Mary Elizabeth Langer donned the black robe of a judge.

She was formally installed last Friday as the newest judge on the Richmond Juvenile and Domestic Relations District Court. She succeeds Judge Angela E. Roberts, who retired July 29 after 26 years on the bench.

Judge Langer was sworn in by her friend and mentor, retired Richmond Juvenile Court Judge Kimberly B. O’Donnell, in a packed second floor courtroom in the John Marshall Courts Building.

In her remarks, Judge Langer promised to use “fairness, compassion and kindness” to deal with those who come before her and to continue the collaborative approach that was a hallmark of Judge Roberts’ tenure.

She is one of four full-time judges on the court that handles a broad range of issues involving families and children, ranging from juvenile offenders to domestic abuse and adoptions. The court now includes only one African-American, Judge Marilynn C. Goss.

The installation of the new judge was bittersweet for her former boss, Richmond Commonwealth’s Attorney Michael N. Herring.

While he is proud of her selection, he said his office “has lost a reservoir of knowledge” on issues involving families and children, which was her specialty.

Judge Langer served as deputy commonwealth’s attorney for 10 years. Mr. Herring said that she often argued with him to keep juvenile offenders out of the adult courts except as a last resort or when a juvenile was accused of a particularly heinous crime.

“I often disagreed with her,” he said, “but I’ve come around to her point of view.”

Along with friends and family, 20 current and former judges from Richmond and other localities were on hand to witness her installation.

Judge C.N. Jenkins Jr., chief judge of the Richmond Circuit Court, presided over the ceremony in which Delegates Manoli Loupassi, R-Richmond, and Jennifer L. McClellan, D-Richmond and state Sen. Glen H. Sturtevant Jr., participated as did Richmond Sheriff C.T. Woody Jr.

A native of White Hall, Ill., Judge Langer began her legal career after earning a bachelor’s degree from the University of Notre Dame and a juris doctor from the Boston College of Law.

Now 52, she has represented defendants and prosecuted them. She started as a public defender in Richmond and then spent seven years with the Chesterfield County commonwealth’s attorney’s office before joining Mr. Herring’s office in 2006.

Judge Langer said she has been used to speaking in court as an advocate, but in her new role, she will be doing more listening in seeking to make sound decisions.