Superintendent Carol R. Johnson announced today she would step down after nearly six years at the helm of the Boston school system.

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She said in a statement that it was a “difficult decision” to leave the job she has held for nearly six years but the loss of her husband, Matthew, had been “life-altering for me and my entire family.”

She said she was proud of her accomplishments since she took office in August 2007. She cited, among other things, improved high school graduation rates and MCAS performance, bringing hundreds of students back to school who had dropped out, and closing achievement gaps.

“We are proud to have helped more students enter and complete college,” she said in a statement posted on the Web.

“Dr. Johnson is one of the most compassionate, caring and talented Superintendents in the United States,” Mayor Thomas M. Menino said in a statement. “She continued the extraordinary transformation of our schools and from day one has focused on creating better schools and offering great classrooms for every child. I often say that she has one of the hardest jobs in the city and she has done it well. We are grateful for everything she has been able to accomplish for our city’s families.”

Matthew Johnson died in March following a lengthy illness. Johnson, who resided in Tennessee, had been ill for several months. The superintendent had repeatedly traveled there to be with him. The two had been married for nearly 40 years and had three children together. Like the superintendent, Matthew Johnson was an educator.

Carol Johnson has informed the school committee that she expects to retire in July. Her contract runs through June 2015, so she will ask the school board to accept her retirement and appoint an interim superintendent, the school system said in a statement.

“Our Superintendent deserves our thanks for all she has done to further a school system that offers a great education for every child,” School Committee Chairman Michael O’Neill said in a statement.

The Boston public schools serve more than 57,000 students in 127 schools.

On Tuesday Johnson had spoken as if she was going to remain in her post, telling the Globe that she was committed to increasing diversity of the teaching force in Boston. She also defended the integrity of the teacher evaluation system, arguing that it is far more objective than the old system, which often failed to define expectations for teachers.

Johnson was reacting to the assertion by the Boston Teachers Union that under the new evaluation plan black teachers were three times more likely than white teachers to be placed on a “directed growth plan” or an “improvement plan,” a move that can lead to termination if an evaluator determines a teacher has failed to overcome shortcomings in the classroom.

Similarly, Hispanic teachers are one and a half times more likely than white teachers to be placed on one of those plans, according to the union analysis.