During the first decades of the nineteenth century, Mill Hill remained relatively undeveloped. At this time, it was not yet a part of the City of Trenton. Variously known as Littleworth, Kingsbury, and Kensington Hill, it was generally thought of as part of a section called Bloomsbury. In 1840, the entire area was incorporated as South Trenton. It was annexed to the City of Trenton in 1851.

The name Mill Hill was applied to the area at least as early as 1821, although as yet relatively little beyond the original mill appears to have been built between Broad Street and the Delaware and Raritan Canal. However, a few streets had been laid out, notably Market Street, Livingston Street, Jackson Street from Market to the Assunpink Creek, and what is now Davis Alley behind the properties on Broad Street.

In the late 1830s and 1840s, the opening of the Delaware and Raritan Canal and the Camden and Arnboy and Philadelphia Railroads, providing transportation to both New York and Philadelphia triggered industrial development on the periphery of the district.

By 1849 there were a rope walk, a lime kiln, and factories manufacturing fire brick and candles. By this time the original Stacy grist mill had been rebuilt as a paper mill, and an amusement park called Washington Retreat had been opened north of the mill along the Assunpink. Owned by Andrew Quintin, it featured a bowling alley, rifle gallery, soda fountain and baths.

Mill Hill grew rapidly as a residential area through the second half of the nineteenth century, with some decline towards the end of the century. City directories for the period list the following number of households: 1854, 128; 1865, 267; 1875, 194; 1885, 259; 1895, 181. The directories also reveal a good deal about the social composition of Mill Hill. Quite clearly, it was a middle class neighborhood. The population was predominantly made up of small tradesmen and skilled industrial workers, with a smattering of professionals. The numbers are fleshed out by information about the activities of some of the men who lived in Mill Hill.

Most of the buildings on Mercer Street, with the exception of the Friends Meeting House, were erected in the last three decades of the nineteenth century. Judge George W. McPherson recalled, “My father moved with his family from Front Street to Mercer Street in the winter of 1864. Mercer Street at that time was not fully built up. The only house from the Creek to Market Street on the east side was a row of four or five houses…”

The growth of Mill Hill required an improved road system. New bridges were erected over the Assunpink. A stone bridge, built between 1836 and 1849, connected Montgomery and Mercer Streets. This was surmounted by ornamental cast-iron railings in 1873. The Jackson Street crossing was spanned by a Pratt truss bridge, constructed by the New Jersey Steel and Iron Co. in 1888. In the 1850s sidewalks were required on Jackson, Mercer and Livingston Streets, and a vitrified brick pavement was laid on Jackson Street in the 1890s.