Although historically not a true manor house, what is today known as "Glenelg Manor" forms the nucleus of a sprawling structure, which houses the Glenelg Country School elementary division. The "Manor House" portion was itself built in two sections: the old "L" wing dating from the middle or late 1700s, and the newer front section built circa 1845. The original house is believed by many historians to have been erected by Ephraim Howard, son of Sarah Dorsey Howard who received the land from her father, John Dorsey, in 1735 as part of her dowry. Sarah's husband, Henry Howard, called the property "Howard's Resolution."

General Joseph Tyson, Assistant Postmaster General under President Tyler, added the newer part of the mansion, which is of Norman Tudor style, circa 1845. The massive granite blocks used as decoration in the addition were quarried near Ellicott City, while the marble at the entrance and in the fireplaces was imported from England. The spacious rooms were of such dimension that each had one or more fireplaces. General and Mrs. Tyson called their home "Glenelg" after an old estate in Scotland and "because it spelled the same from either end."

In 1915, Glenelg Manor was purchased by Mr. and Mrs. W. Bladen Lowndes (he being a Baltimore attorney) who made it their country home until the death of Mr. Lowndes in 1941. Mrs. Lowndes was responsible for developing the grounds until they were counted among the most beautiful in the state. An original hedge of old boxwood 400 feet long inspired the design of the formal gardens. A woodland bridle path, bordering on a stream and enhanced by shrubbery and wild flowers, was developed and a tennis court, six-hole golf course and swimming pool provided additional recreation for the family's many noted guests.

Mr. Zaizers purchased the property in 1941 from Mrs. Lowndes and then sold it to his brother-in-law, Mr. Maisel in 1960.

The original building was rented in 1954 when Glenelg Country School was founded by Kingdon Gould, Jr. and his wife Mary Thorne Gould, along with Mr. and Mrs. John T. Mason, Jr., Judge James Macgill and Mr. and Mrs. William Shippen. Originally, the school included 35 students, enrolled in grades K through eight, and a faculty of five. Miss Marjorie Dunn was the first Headmistress.

The school leased the Manor House until 1963 when the premises and 50 acres were purchased and a major addition of classrooms, offices and gymnasium was built connecting the Manor with the old carriage house/dormitory. An option to purchase an additional 30 acres was exercised in the late 1970s. The first grade addition to the primary wing was added in 1975 and dedicated to Royden Blunt, a trustee and major donor. In 1979 a new gymnasium was built and the old one converted into classrooms, new offices, a science lab and a commons area. The current swimming pool was completed in the summer of 1983 for use of the Summer Camp Program. Expansion to the secondary level began in the fall of 1985 with the ninth grade lodged in a temporary classroom building behind the Manor House. The secondary division building was completed and in use by September 1987, occupied by seventeen students in ninth grade and nine students in tenth grade. The building was dedicated to founders Kingdon and Mary Gould, in honor of their service and loyalty to the School. The first senior class graduated in June 1989.

As a result of continued enrollment growth and as a part of the Long-Range Plan, a Middle School building and a Performing Arts Center were added to the campus in the fall of 1997. Today, the Middle School has grown to four sections for each grade. The Performing Arts, its centerpiece being the Mulitz Theater, also provided growth in the arts area and is used by all divisions of the School. Visiting professional groups, such as the National Players are also featured in the Mulitz Theater. In January of 2002, the school opened a Primary Building for grades Pre-K through 1. The most recent addition to the campus is a 25,500 square foot academic expansion in the Upper School for library, technology and science, and a 35,000 square foot athletic center which opened in January 2005.

The enrollment as of September 2007 is 806 students

The philosophy of the Glenelg Country School upon its founding in 1954 was to provide a non-sectarian, co-educational, college preparatory education for Howard County students. Since that time, Glenelg has provided a learning environment which fosters academic skills, intellectual development, and personal growth. The same philosophy and practices that have made Glenelg a respected and successful academic institution also distinguish its summer programs. Excellent teachers, a nurturing learning environment, individual attention, and small classes have long been hallmarks of the School.