It's a bit confusing. The territory of Delaware was originally held by various other colonies. There was a Dutch attempt, circa 1631, that vanished. Then the Swedes created New Christiana, around the Delaware River valley, and that was taken over by the Dutch and made part of New Netherland circa 1655. That was in turn taken over by the British in 1674, after the Anglo-Dutch War. There was then an almost comic period after 1681 in which the area had overlapping claims by both Maryland and Pennsylvania and residents in some counties were approached and threatened by tax collectors from both colonies. Within a few years, it was mostly given over to Pennsylvania...but the Penn and Calvert families would dispute claims for much longer than that ( the background to John Barth's comic novel, The Sotweed Factor).

Pennsylvania was a proprietary colony of the Penn family. Most of the 13 colonies had begun as proprietary colonies ( basically owned by a family, like the Penns, or company, like the Virginia Company) , but Pennsylvania was unusual in that it had not over time been made into a crown colony, under state control with a royal-appointed governor: it stayed a province of the Penn family. Ben Franklin was sent to Britain in 1757 to try to convince them and the British government to let the crown take it, but the Penns refused- they were making money off the place.

So, in effect the state later known as Delaware seceded both from the Penn family's colony and from Britain to become its own state, in 1776, never having been its own colony. Though not a named colony, there were certainly colonists there. This was also before the Articles of Confederation, which laid out more rules as to how the states would interact, and well before the Constitution, which stated that new states could not be created from an existing state without the prior agreement of that existing state.

So, Delaware's colonial history is complicated, and I'm going to lean heavily on Dr. John A. Munroe's Colonial Delaware: A History, as I think it's stood the test of time and is still the best discussion of later colonial history of Delaware.

I’ll provide my usual caveat that I’m sure there other people who can answer this better than I can on this sub.

I'll add to u/Bodark43's answer that after 1674 when the English were firmly in control of the area again after the Second Anglo-Dutch War, the area was attached to the Duke of York's grant that would eventually become the colony New York, but was essentially ministered by as a separate colony, for two reasons: Firstly, it was far away from New York, and secondly- it hadn't really had been explicitly granted to the Duke of York when King Charles II granted his brother the right to conquer the Dutch colonies in North America. This lead to the Calvert family claiming the entire Delmarva peninsula (and the start of the Penn/Calvert family disputes that would continue right up until 1772), and attempting to make this claim stick by burning the Mennonite colony at Whorekill (modern day Lewes, Delaware) in 1673. The colonial administration of the Delaware valley at this point was a hybrid of Dutch and English court systems, with a English man serving essentially as a schout in the Dutch style, but the area divided into counties in the English style.

Then, in 1681 William Penn was granted the lands starting north 40 degrees of Latitude north from the edge of the edge of New Castle County (an undefined border to begin with!), but worried that his new colony, that was supposed to be a refuge for English Quakers and nonconformists, would be landlocked. Thankfully, William Penn's father had been a close companion of the Duke of York, and in 1682, he agreed to grant William Penn his lands on the west side of Delaware Bay.

William Penn took possession of his newly granted colony in 1682, first landing in New Castle (in modern Delaware), and attempted to gather his very diverse new colonists into something resembling a cohesive body. At this point, the area that he had been granted included not only the Native Americans that had lived there for several thousand years, but Swedes, Finns, Dutch, English, Scottish and both free and enslaved African peoples. He created a colonial government that included two legislative bodies and a governor to oversee his interest as proprietor.

From the start, there were issues. The areas north of New Castle along the Delaware had been sparsely populated before Quaker settlers started arriving, and were very quickly majority English Quaker. In the southern three counties (we will circle back to that theme- remember that), there were already more settlers, and most of them were either Swedish or Finnish Lutherans, Dutch Calvinists, or English Anglicans- they were already suspicious of the English Quaker newcomers. Only one year after Penn had arrived, the elected legislative representatives from Kent and Sussex counties didn’t come to Philadelphia for the second session of assembly.

The next few years were rocky, in terms of the relations between the “upper counties” (Bucks, Philadelphia, and Chester) and the “Lower Counties” (New Castle, Kent, and Sussex)- as neither group wanted to meet closer to the other, they argued over tax collection, and they argued over representation. The three upper counties were growing rapidly, and resented the fact that each county had equal representation. For their part, the lower three countries wouldn’t settle for anything less than equal representation for each count.

There was also an issue unique to Penn’s colony: Pacifism. The Quakers, being a sect that was (and still is) deeply committed to pacifism, did not set up a militia as most other colonies did in this period. As piracy and privateering became a bigger issue in the 1690s in the Delaware Bay, representatives from the lower counties started to complain they were exposed to danger and the Quaker governors and other assemblymen wouldn’t pay for defenses for the lower counties, or allocate money for any defense measures.

In 1701, this came to a head, and William Penn was forced to grant a new charter to his colony, which permitted a division of his colony into two parts if the colonialist wanted it. In 1704, this eventually came to a head, and separate legislative assemblies were created for Pennsylvania and the “Lower Three Counties” on the Delaware Bay. The two polities would share a governor (either a member of a Penn family or their representative), but administer themselves as separate colonies.

The Lower Three Counties on The Delaware were thereafter run as a subsidiary of Pennsylvania with a separate assembly, a separate militia (As Pennsylvania didn’t have a regular militia until 1777), and separate lower courts (the upper courts were on a circuit across Pennsylvania and the Lower Three Counties). As with Pennsylvania, the colonialists in Delaware had their issues with the boundary with Maryland and with the proprietary government of the Penn family that continued through the 18th century.

As mentioned above, Delaware did declare independence from both Pennsylvania and Crown on June 15th, 1776, a few weeks before the Continental Congress in session at Philadelphia declared independence from Great Britain, the same day they retracted their instructions to their representatives to the Continental Congress to vote against independence. I think Munroe interprets this correctly as a move by the Delaware assembly to be seen as in agreement with their larger neighbors, and an attempt to keep the independence from Penn family and royal control they had carved out for themselves over the last seven decades.

Whether or not Delaware is a “separate colony” or not really comes down to how you define a “colony”. Was Delaware founded as a separate colony by the English? No. However, I think Delaware would qualify, as it had its own legislative body and separate laws from Pennsylvania, and was treated as a separate entity by most people in the period.

Sources cited:

Munroe, John A. Colonial Delaware: A History. Wilmington, DE: Delaware Heritage Press, Delaware Heritage Commission, 2003.

Although dated, I think this is still the best introduction to Delaware’s colonial history from 1674-1776.

Rodney, Richard S., and Burton Alva Konkle. Early Relations of Delaware and Pennsylvania. Freeport, N.Y.: Books for Libraries Press, 1971.

This is also a good summary of the legal breakdown of Pennsylvania into Pennsylvania and The Lower Three Counties.