Martin Luther King, Jr. Middle School (formerly Mosby Middle School) opened its doors to welcome students and its community in September 1964. It was originally designed for a capacity of 1,500 junior high pupils and 1,000 elementary pupils. The elementary grades were discontinued in 1970 when it became Mosby Middle School. In 2004, Mosby and Onslow Minnis, Jr. Middle schools were combined.
A new name was selected, Martin Luther King, Jr. Middle School. The original Mosby Middle School was named for Benjamin Mosby, a realtor who developed the area early in the nineteenth century. Part of this area became known as Union Hill and part as Howard's Grove, in which (prior to annexation by the city in 1906) a four-room high school was maintained by Henrico County. According to the Richmond Civil War Centennial Committee, the school grounds occupy part of the site of Howard's Grove, a popular recreational area which during the American Civil War became one of the largest Confederate hospitals.
The first principal in the new building was Dr. Arnold R. Henderson, who served until 1966, when he became principal of Maggie L. Walker High School. Mr. Fred Swann succeeded Dr. Henderson and remained at Mosby until 1975. Dr. Joe Louis Simmons followed as principal until 1980; he was followed by Mr. Lyman Lloyd who served until 1984, when Mr. John Lane was appointed. Mr. Lane held the position through the 1984-1985 school year. Dr. Roy West followed Mr. Lane as principal from 1985-1989. In September 1989, Dr. West's tenure was followed by the appointment of Dr. Ronald L. Carey, former principal of Swansboro Elementary School. In 1991, during Dr. Carey's tenure, East End Middle School and Mosby Middle School were consolidated to form the newly combined Mosby Middle School.
In 1995, East End Middle School (renovated and renamed Onslow Minnis Middle School) reopened once again as a separate middle school. In 1998, Dr. Carey's appointment as Assistant Superintendent of Elementary Education for Richmond Public Schools led to the arrival of Mr. Leroy Diggs as Mosby’s next principal. Formerly one of the assistant principals at John F. Kennedy High School, Mr. Diggs became principal of Mosby in1998, and retired at the end of the 2001-2002 school year. Mr. Aaron Dixon, former assistant principal of the Adult Career Development Center (ACDC) was appointed principal of Mosby Middle school in 2002, and remained until the close of the 2010-2011 school year. Dr. Valerie Harris succeeded Mr. Aaron Dixon from September 2013 until February 2014. Shortly there after, Rickie Hopkins assumed the leadership role until June 2015. Mr. Derek Scarborough was appointed as principal in the summer of 2015 until his departure in June 2016. As of July 1, 2016 Dr. Jeryl Scott was appointed as the new principal, until her departure in June 2017. As of June 23, 2017 Inette Dabney was appointed as the new principal of Martin Luther King Jr Middle School.