Dayville was originally called “Daysville” after Captain John Day, who owned and developed the water privilege along the Assawaga (Five Mile) River. It became the freight hub for other manufacturing sites along the river and Whetstone Brook with the construction of the Norwich & Worcester Railroad in the 1830s. The mills picked up the cotton at the Dayville depot, hauled the bales to their factories and shipped the finished goods the same way.

Dayville was more than simply a mill town as there are houses associated with merchants, carriage makers, machinists, carpenters, doctors, lumber dealers and others who made Dayville a commercial as well as an industrial center.

John Day, a well-to-do farmer with interests in real estate, built the Sayles Mill in 1846. Sabin and Harris Sayles built numerous mill houses for the use of their larger work force after building a new brick mill in 1882. This enabled the work force to be within walking distance of the factory. They also built a company store and a meeting hall in the large brick block (c. 1873) in the center of Dayville.

For ten years after the Sayles Company, the mill was operated as the Dayville Woolen Company. They sold out to the Assawaga Company in 1903 that prospered during the World War I period. Textile production came to an end in the Depression but wire manufacturer William Prym bought the Assawaga Woolen Mill in 1939. The company manufactured small metal notions, common pins, snap fasteners, hooks and eyes, etc.; some designed and invented by Herman Kohl.

The old Dayville School has now been replaced by three modern schools—Killingly Central and Killingly Intermediate Schools and the new Killingly High School opened in September 2010.

The popular 20th century summer resort known as Wildwood Park at Alexander’s Lake was owned in the 18th and 19th centuries by the Alexander family. The east side of the lake was developed as a resort in 1890 by the People’s Tramway Company on leased land. In 1919 the lake and acreage were sold to Patrick Sheridan, father of John Sheridan who owned the property until his death. Trolley service to Wildwood Park was provided by the People’s Tramway Company and also the Providence & Danielson line that terminated in Elmville. A transfer station was built across the street from the Lake so passengers could change to the north-south line. The People’s Tramway Company was a subsidiary of the People’s Light & Power Company. It was common for electric companies to expand into electric street railways or traction companies as they were sometimes called. Wildwood Park became a popular spot for day trips. Trolleys made their first stop at 5:24 a.m. and stopped every half-hour all day in the summer.

The Elmville Stand was the connection of the Providence & Danielson electric railroad and the People’s Tramway Company that ran from Danielson to Putnam. The Providence & Danielson was a passenger and freight line. A freight and milk car made one trip a day from Danielson to Providence. After the line was abandoned in 1920-21, the old waiting station was used for years as an ice cream stand and lunchroom.

In 1933 Danielson Manufacturing Company (Danco), a manufacturer of mill supplies and later injection molded nylon products occupied the factory at the foot of Dog Hill where Colt’s Plastics is now situated.

The oldest extant structure in Killingly Center, built about 1790, was known in its early years as the Jeremiah Field Tavern, a stagecoach inn now divided into apartments. George Kingsbury operated it in 1839 when it was known as the Kingsbury Inn. The Killingly Grange #112 now occupies the old Killingly Center School, c. 1848

The Killingly Center area became active in the mid 18th century when the Breakneck Hill Congregational Church was moved to the intersection of Cook Hill Road and Route 101 to be used as the Town Meeting House. The first town meeting was held there on December 12, 1785, and continued in use for Killingly Town Meetings until 1906 when the town purchased the “Music Hall” in Danielson. The old building was demolished in 1933 after being sold by the town in 1922.

The area surrounding the junction of Routes 12 and 101 is the site of three shopping plazas containing a variety of stores and food services making it a thriving commercial area, the latest known as Killingly Commons. At one time a major glass manufacturing concern was located here. Killingly’s Industrial Park is just off Exit 94 of Route 395. Over 3000 people are employed at over 15 manufacturing establishments here. Frito-Lay is the town’s largest employer of over 700 people.