On 24 April 1771, three Quaker brothers from Bucks County, Pennsylvania, north of Philadelphia, chose the picturesque wilderness several miles upriver from Elk Ridge Landing (today's Elkridge, Maryland), the uppermost part of the river then navigable by tobacco-loading sailing merchant ships in the 18th century, to establish a flour mill, purchasing 50 acres (20 ha) of Baltimore County land from Emanuel Teal and 35 acres (14 ha) from William Williams.

[5]: 12  They helped revolutionize farming in the area by persuading farmers to plant wheat instead of tobacco and also by introducing Plaster of Paris fertilizer to revitalize depleted soil.

[8] Charles Carroll of Carrollton (1737–1832), the last surviving signer of the Declaration of Independence, a rare Roman Catholic and a wealthy landowner with the largest fortune then in colonial America, was an early influential convert from tobacco to wheat.

In 1829, New York industrialist and Baltimore foundry-owner Peter Cooper began testing his iron steam engine, Tom Thumb (1791–1883), on the B&O Railway.

The famous race between Tom Thumb and a horse-drawn rail carriage took place between Relay Junction on the return trip from Ellicott's Mills towards Baltimore in August 1830.

Even though the horse won the race due to a sudden broken drive belt on the Tom Thumb, it heralded the time when steam engines steadily improved, and the soon-to-be steam-operated railroad became a vital link in the town's economy and later expanding to the city of Baltimore's economic supremacy along with the state in the nation.

[13] Later that month, Union Army troops seized the "Winans Steam Gun" which had been en route to Harpers Ferry, Virginia, at Ellicott's Mills.

[14]: 22  In the fall of 1862, the 12th New Jersey Volunteer Infantry was assigned to guard Ellicott's Mills, setting up the 1,200-man Camp Johnson on the lawn of the nearby Patapsco Female Institute.

The one-day delay by Wallace's small force at Monocacy Junction enabled Lt. Gen. Ulysses S. Grant to rush troops in time to defend the U.S. capital.

Talbot relocated uphill to a brick and granite store designed by Charles Timanus that houses the Ellicott City Brewing Company today.

[19] In 1879, political gangs controlled the polling locations, shooting and wounding African American Ellicott City voters.

[20] In H. L. Mencken's best-selling memoir Happy Days, 1880–1892, he described his childhood in the chapter "Rural Delights" while living with his parents in their rented home in Ellicott City.

[21] Ellicott City favored the temperance movement, enacting a law against "spiritous, fermented or intoxicating liquors" in 1882, taking effect May 1, 1883.

This was shortly changed to limit sales of liquor to licensed shops that did not sell other goods, providing the primary source of the town's tax income.

[35] Alda Hopkins Clark purchased the Ellicott City First Presbyterian Church to donate it to the Howard County Historical Society.

[18]: 108 The same year, the state health department ordered the city to stop dumping its raw sewage into the Patuxent River and develop a modern septic system.

[36] In 1964, the Corinthian Conservation Company was proposed to operate a Title I private-nonprofit partnership to implement a slum eradication program in Ellicott City, demolishing residences, and replacing them with 75% federal-funded apartment complexes.

[41] In 2011, a portion of the former 83-acre (34 ha) landfill site was developed with a $462,000 grant from the Maryland Energy Administration to build onsite solar arrays to power Worthington Elementary.

Many of the attractions have been moved to Clark's Elioak Farm in a rural area in the southwest corner of the Ellicott City CDP, where they are being restored.

[50] In 2014, the Hiene House and Ellicott City Jail were placed on the Preserve Howard top-ten most endangered list due to walkway and parking lot construction plans.

[52] Ellicott City has been home to a large Korean population along its Route 40 corridor, where numerous Korean-owned businesses and restaurants operate.

[66][67] The removal of nine or ten historic buildings was opposed by preservationists as well as residents and Democrat Calvin Ball defeated incumbent Republican Allan Kittleman as county executive.

Several deep stream valleys converge at this location, which increases the risk of flooding, but at the same time creates the town's heights.

[81] The Howard County Tourism Council runs a Ghost Tour that visits several places with reputations for paranormal activity.

Ida; the B&O railroad bridge that crosses Main Street in the center of the town; the old Ellicott City Firehouse; and the Patapsco Female Institute.

Since 2023, Ellicott City, along with the rest of Howard County, has been a part of Maryland's 3rd congressional district, represented by Democrat John Sarbanes.

The Howard County Circuit Courthouse, originally located on Court Avenue on a hilltop north of the downtown area, moved in 2021 to a new building adjacent to the Columbia Hills neighborhood.

[98] The Homewood Center and the system's other specialized school, along with the central offices, also have Ellicott City addresses, though in fact they are on the northern edge of Columbia.

[98] Middle schools serving the CDP are Burleigh Manor, Dunloggin, Bonnie Branch, Mount View, Folly Quarter, Ellicott Mills and Patapsco.

Ellicott City is served by the Regional Transportation Agency of Central Maryland (RTA) by Route 405 (Yellow Line) travelling from the Columbia Mall to the Miller Branch Public Library.