Baldwin was born to Peter L. and Mary E. Baldwin, and received all of her education in Cambridge’s schools. In 1874, Baldwin graduated from Cambridge High School, and a year later she graduated from the Cambridge training school for teachers.
Baldwin first taught in Chestertown, Maryland, for two years. In 1881, she was hired as primary-grade teacher at the Agassiz Grammar School of Cambridge. In 1889, she became principal of the school, making her the first African-American female principal in Massachusetts and the Northeast. As principal, Baldwin supervised white faculty and a predominantly white student body. In 1916, as a new Agassiz school was erected to include higher grades, Baldwin was made master, supervising twelve teachers and five hundred students, all whites. She was one of only two women in the Cambridge school system who held the position of master and the only African American in New England to hold such a position.
Baldwin served as master of Agassiz school for forty years. Under her leadership, the school of Agassiz became one of the best in the city, attended by children of Harvard professors and many of the old Cambridge families. She introduced new methods of teaching mathematics and began art classes. She was the first to introduce the practice of hiring a school nurse. Her school was the only one in the city of Cambridge to establish an “open-air” classroom.