Once part of Algonquian territory, Brookline was first settled by European colonists in the early 17th century. The area was an outlying part of the colonial settlement of Boston and known as the hamlet of Muddy River. In 1705, it was incorporated as the independent town of Brookline. The northern and southern borders of the town were marked by two small rivers or brooks, hence the name. The northern border with Brighton (which was itself part of Cambridge until 1807) was Smelt Brook. (That name appears on maps starting at least as early as 1852, but sometime between 1888 and 1925 the brook was covered over.) The southern boundary, abutting Boston, was the Muddy River.
The Town of Brighton was merged with Boston in 1874, and the Boston-Brookline border was redrawn to connect the new Back Bay neighborhood with Allston-Brighton. This created a narrow strip of land along the Charles River belonging to Boston, cutting Brookline off from the shoreline. It also put certain lands north of the Muddy River on the Boston side, including what are now Kenmore Square and Packard’s Corner. The current northern border follows Commonwealth Avenue, and on the northeast, St. Mary’s Street. When the Emerald Necklace of parks and parkways was designed for Boston by Frederick Law Olmsted in the 1890s, the Muddy River was integrated into the Riverway and Olmsted Park, creating parkland accessible by both Boston and Brookline residents.
Throughout its history, Brookline has resisted being annexed by Boston, in particular during the Brookline-Boston annexation debate of 1873. The neighboring towns of West Roxbury and Hyde Park connected Brookline to the rest of Norfolk County until they were annexed by Boston in 1874 and 1912, respectively, putting them in Suffolk County. Brookline is now separated from the remainder of Norfolk County.