The first settlers in 1834-1835 of Rockford, Illinois were Germanicus Kent, Thatcher Blake, and Lewis Lemon, who came from Galena and established themselves on the west bank of Rock River; and Daniel Shaw Haight, who founded a settlement on the east bank. The community was briefly known as "Midway", but soon became known as "Rockford", because of the excellent ford across Rock River. The settlement was incorporated as a village in 1839, and chartered as a city in 1852. A post office was established in 1837 and the first weekly newspaper was published in 1840. Rockford Female Seminary was chartered in 1847, became Rockford College in 1892, and became fully coeducational in 1958.

The earliest settlers were chiefly from New York state and New England, but the city acquired a modest cosmopolitan character. Large numbers of Irish born immigrants arrived in the 1850s, and a few Swedish immigrants came in 1852 but very soon the Swedes began to come in large numbers and quickly became the largest ethnic group in the city. They settled chiefly on the east side and the Swedish language was as common as English. Other significant ethnic groups that had a presence in Rockford were the Italians after 1880, the Poles and Lithuanians after 1900 and Laotians, Vietnamese, and Hispanics after 1970. The black population of the city was very small until the First World War, after which people from the south, particularly Arkansas and Mississippi, arrived.

Until the 1850s Rockford was a sleepy country village. But in 1851 the Rockford Water Power Company was organized and in 1852 the Galena and Chicago Union Railroad (now part of the Chicago & Northwestern system) reached the city. By 1860 the city had become a significant, growing industrial center, noted for production of agricultural machinery. By the 1880s a furniture industry began, using the talents of Swedish craftsmen, and in the first half of the 20th century Rockford was the second largest furniture-manufacturing center in the United States. However, by the First World War, the agricultural implement industry was in decline and the furniture industry was severely damaged by the Great Depression and the Second World War. By the end of the 1960s both were extinct in the city.

The city’s industrial background has produced many important and interesting inventions, among them are the knitting machine for socks, the airbrush, the electric brake, the electric garage door opener, electronic dart board and the dollar bill changer. Rockford’s 20th century industry revolves around machine tools, heavy machinery, automotive, aerospace, fastener and cabinet hardware products, and packaging devices and concepts.

Before the Civil War Rockford was the center of an avid anti-slavery region. In 1845, representative Anson Miller introduced a bill in the state legislature with an impassioned speech in an unsuccessful attempt to repeal the Illinois "Black Laws." After the Civil War, the Illinois Supreme Court, Chicago, decided an important civil rights case from Rockford. The decision supported African Americans’ rights to unhindered use of public transportation, but was decided on such narrow legal grounds it was cited in support of the "separate but equal" doctrine established by the United States Supreme Court in Plessy v. Ferguson (1896).

In the six years following the Civil War Rockford became nationally known in baseball circles, with the remarkable success of its Forest City Baseball Club. The team had become the most prominent western club and joined the first professional baseball league, the National Association, for the 1871 season. The city was a member of the Northwestern League, the first minor league, in 1879, and since that time has frequently had a minor league club. During the existence of the All American Girls Professional Baseball League, 1944-1954, the city supported one of the league’s dominant clubs, the Rockford Peaches, the team featured in the 1992 movie A League of Their Own.

Rockford’s municipal politics in the 19th century often revolved around liquor issues, the "drys" and the "wets" often being about equal in electoral strength. The early 20th century saw the rise of organized labour as a potent force locally.

When the war broke out in 1861, one of the first Illinois military contingents to be mobilized was the Rockford Zouaves. In the first half of the 20th century Rockford was an important military city. Camp Grant was opened south of the city in 1917, and thousands of soldiers were trained here for World War I. The camp was deactivated in 1921 and in 1924 was turned over to the state of Illinois as a permanent training center for the National Guard. In 1940 Camp Grant was reactivated as a United States Army base and served as a reception center for draftees, a training center for medical corpsmen, and finally a separation center. Between 1943 and 1946 German prisoners of war were housed at the Camp, to a maximum number of just over 2,000. Camp Grant was deactivated in 1946 and most of its territory is now divided between the Greater Rockford Airport and Seth Atwood Park.