This track was built in 1920 for the first Kent and Sussex County Fair. Harness racing and other races were conducted annually as part of the fair (which changed its name to the Delaware State Fair in 1962). In 1934, the fair became an organization to coordinate harness racing dates at fairs in the Mid-Atlantic region. This organization is called the Central Fair Circuit. Then in 1945, the directors of the fair organized the Kent and Sussex Racing Association to conduct an annual racing meet. It happened after the state legalized pari-mutuel wagering on harness racing. Its first 15-day meet with pari-mutuel betting occurred on October 14, 1946.

In 1964, the racing association began planning to renovate the facility to match the weather to give a comfortable environment for the spectators during the colder autumn months, when the racing meet was held. Plans were formalized in 1966 to construct a new track to the south of the existing facility. However, the high cost canceled the plan. Instead, a $1-million project was undertaken to build an enclosed clubhouse to enhance the grandstand, including improved heating. Along with the improvements, the track’s name was changed in 1967 from the Kent and Sussex Raceway to Harrington Raceway. In 1968, the construction was completed.

In 1994, Delaware legalized slot machines at horse tracks to save the racing industry. In March 1995, the majority of the fair board of directors voted to build a slot parlor. Then Harrington’s slot machine parlor, Midway Slots, and Racebook opened on August 20, 1996. The track was renovated in 2003 to widen it and make the turns gentler, making it safer and more attractive to horse riders.

In 2006, a $50-million expansion and renovation of the slot parlor commenced. As part of the project, the Midway Slots name was dropped in 2007, and the entire property became known as Harrington Raceway & Casino. Full House also ceded day-to-day operations of the gaming facility to Harrington. The expansion was completed in 2008, adding a new restaurant, lounge, coffee shop, high-stakes gaming areas, and an expanded buffet, and increasing the number of slot machines in the casino from approximately 1,600 to 2,100. Table games were added in 2010, following their legalization earlier in the year. The management agreement expired in 2011, leaving Harrington Raceway in full control of the casino.