Washington Street in Boston just seems to go on forever, and for good reason. Washington Street was the only road leading out of Boston (this map shows Boston and its relationship with surrounding towns--zoom in to see all kinds of detail) until the opening of the bridge to Charlestown in 1786, so to get to Dorchester, or Roxbury, or Brookline, or anywhere really, you were taking the long road to the mainland. As noted below, Washington Street, until 1788, had different names for different sections—the old Cornhill, Marlboro, Newbury, and Orange Streets were renamed to make Washington Street. I’m a little puzzled about why one street had so many different names. If you’ve walked that stretch of Washington Street, you know that it is actually quite short. My hypothesis is that at a time when house numbers were not in use, breaking the same street into several sections with different names helped people locate homes and businesses more easily.
Readers will also note that after Boston annexed Roxbury in the 1860s, a good deal of effort was required to harmonize the street patterns of the formerly distinct entities. A few other points worth mentioning: it seems as if part of Washington Street in Roxbury ran close to the street now known as Malcolm X Boulevard (New Dudley Street), which then changes name at the Roxbury Crossing Orange Line stop to Tremont Street, which then continues on to Huntington Ave (Route 9, formerly known at the Worcester Turnpike. This would place Wait’s Mills near the location of Roxbury Crossing, which makes sense because this would be where Washington Street crossed the Stony Brook.
I just walked over and picked up Francis Drake’s The Town of Roxbury, which confirmed my analysis: “Here, in 1633, the first mill was built in Roxbury by Richard Dummer….For more than a century the Pierpont family were its proprietors, and as quite a settlement grew up around it, the locality acquired the name by which it was long known, of “Pierpont’s Village”. Early in the this century [the 19th, ed.] it was known as “Waitt’s Mill,” the owner being Samuel Waitt, who also occupied an adjoining building for the manufacture of leather breeches…These old buildings, together with the dam, were removed in 1870. The old mill-pond is dried up, the waters of the Stony River now flow through the sewer….” (Drake, The Town of Roxbury, p. 319-20) The railroad would have run along the Stony Brook corridor as well, which, as railroad traffic increased, would have created a need to raise Washington Street above the railroad tracks. This c. 1775 map from the Library of Congress delineates this area of Roxbury (as does this map). If you locate the Roxbury Meetinghouse, you will see two roads leading to the top of the map (and note the indication of "stone" I believe it is this stone). The one to the right leads to the area of Wait's Mill, and if you zoom in you can see the locations of the Mill and other buildings. This road leads to Cambridge and Watertown, while the road to the left leads to "Jamaica", better known today as Jamaica Plain.
From Annual Report of the Street Laying Out Department for the Year 1894:
Washington Street, Boston, Roxbury, and West Roxbury,1788; from Haymarket Square to Dedham Line; from Dock Square to School Street called Cornhill, 1708; from School Street to Summer Street called Marlboro’ Street, 1708; from Summer Street to Essex Street called Newbury Street, 1708; from Essex Street to the fortification near the present line of Dover Street called Orange Street, 1708, from Beach Street to Dover Street having been so called from 1663; from Roxbury line to Orange Street at fortification named Washington Street, by the Selectmen, July 4, 1788; extended northerly through Orange Street, Newbury Street, Marlboro Street, and Cornhill to Dock Square, July 6, 1824; extended from Cornhill to Haymarket Square, November 6, 1872; part in Roxbury laid out from Boston line to Roxbury Street, January 19, 1662; named from Boston line to Dedham and Worcester Turnpikes, May 9, 1825; raised by B & P (N.Y., N.H. & H.) R.R. over crossing near Wait’s Mills, April, 1847; width adopted from Eliot Square to Boston line, April 26, 1852; Norfolk and Bristol Turnpike laid out as a public highway, part in Roxbury named Shawmut Ave. and extended to Boston line to meet end of Shawmut Ave there, June, 1857; so much as lies in West Roxbury named Shawmut Ave., Feb 3, 1858; prospective line established from Lowell (Pynchon) Street (now Columbus Ave.) to Parker Street, July 2, 1860; name of Washington Street from Tremont Street at Pynchon Street (now Columbus Ave.) to Brookline line changed to Tremont Street, April 21, 1868; portion discontinued at junction of Gardner Street, December 15, 1868; name of Washington Street from Guild Row to Pynchon Street changed to Roxbury Street, June 16, 1874; name of Guild Row (formerly Bartlett Street) from Roxbury Street to Dudley Street and of Shawmut Avenue from Dudley Street to Dedham line changed to Washington Street, June 16, 1874.