Calvert is at the intersection of State Highway 6 and Farm roads 1644 and 979, on the Southern Pacific line nine miles north of Hearne in west central Robertson County. The earliest White settler in the area was Joseph Harlan, whose 1837 land grant lay five miles south of what is now the site of Calvert. In 1850 Robert Calvert, for whom the town was named, established a plantation west of the townsite. Calvert and other area farmers urged the Houston and Texas Central Railway to build through the area; the railroad arrived in 1868. A group of investors purchased land at the townsite and platted the community in January of that year, and by February merchants from nearby communities such as Sterling and Owensville were moving to the new town. A post office also opened at the community in 1868. The first trains arrived there in 1869. Calvert incorporated with an aldermanic form of government in 1870. In 1870, as part of the Reconstruction political maneuvering in Robertson County, Calvert replaced Owensville as county seat. Early that year the town was briefly occupied by federal troops; that year also the first school was founded in the community. The Republican party in the county drew much of its strength from Black voters on the plantations in the Calvert area, and for a number of years the party was able to elect Blacks from Calvert to county and state office. As a rail center and as county seat, Calvert prospered, and in 1871 the town claimed to have the largest cotton gin in the world. In 1873 a severe yellow fever epidemic killed many in the community. A county jail built in 1875 was still a local landmark more than a century later.
In 1878 Calvert was a thriving community with some fifty-two businesses. The next year the town of Morgan became county seat, but Calvert continued to prosper as a commercial center. By 1884 Calvert had an estimated 3,000 inhabitants, with Presbyterian, Methodist, Baptist, Episcopal, and Catholic churches, public schools, two banks, an opera house, and the weekly Courier. Around 1900 the community was a major cotton center, with a number of gins, cotton compresses, and cottonseed oil mills (see COTTONSEED INDUSTRY). In 1899 the town was damaged by floods, and two years later a fire destroyed much of its business district. Calvert's population was reported as 3,322 in 1900, but thereafter it began to decline. The community had 2,579 residents in 1910, 2,099 in the mid-1920s, 2,366 in 1940, 2,561 in 1950, 2,073 in 1960, and 1,950 in the mid-1960s. In 1968 many former residents of the town visited to help its citizens celebrate Calvert's centennial. The population was 1,714 in 1980, 1,536 in 1990, and 1,426 in 2000.