The oldest known courthouse on the site dates back to 1687.
The building was originally both a courthouse and a seat of government, as the building was used by the colonial assembly.
During the years when Pennsylvania and Delaware shared an assembly, it generally met in Philadelphia, but occasionally met in New Castle instead, such as in 1684 and 1690.
In 1700, the Pennsylvania colonial assembly met in the recently constructed Court House, with William Penn in attendance.
In addition to the routine passing of laws, Penn's primary focus during the 1700 session was healing a rift that had developed between the assemblymen from Pennsylvania proper and those from Delaware.

This attempt was unsuccessful.
In 1704, after the Pennsylvania counties sought to expand their representation in the assembly, the three Delaware counties demanded, and were granted, an independent assembly so they could pass their own laws.
They chose William Rodney, grandfather of Caesar Rodney, as their first Speaker.
From that year on, the assembly that met in the Court House governed Delaware alone.

This earlier building was mostly destroyed by a fire in 1729 caused by a prisoner's attempt to escape.
The Court House was rebuilt on the same site within a year, and the foundation of the original is still visible.

In 1774, the assembly met here and chose three of Delaware's most famous politicians to represent the colony in the Continental Congress: Thomas McKean, Caesar Rodney, and George Read.
It was in this building that the assembly, at McKean's urging, voted on June 13, 1776 to sever all ties with England as well as the state of Pennsylvania, with which Delaware still shared a governor.
The assembly resolved that:
Whereas it is become absolutely necessary for the safety, protection, and happiness of the good people of this colony, forthwith to establish some authority adequately to the exigency of their affairs, until a new government can be formed; and Whereas the representatives of the people, in this assembly met, alone can and ought at this time, to establish such temporary [authority].
Resolved unanimously, that all persons holding any office, civil or military, in this colony, on the 13 day of June instant, may and shall continue to execute the same, in the name of the
government of the Counties of New Castle, Kent, and Sussex Upon Delaware, as they used legally to exercise it in the name of the King, until a new government shall be formed, agreeable to the resolution of Congress of 15 May instant.

When the Congress issued the Declaration of Independence, it was read to the citizens of the town from the balcony on the second floor.
Later in 1776, Delaware's first Constitution was drafted and adopted by a convention including Read, McKean, and Nicholas Van Dyke inside the Court House.
This was the first adoption of a state constitution following the issuance of the Declaration.
The building served as the state's first capitol until 1777, when Philadelphia and Wilmington were captured by the British.
In light of the danger, Delaware's government moved south, establishing Dover as the new capital city.
The building continued to house both state and federal judicial proceedings until 1888.

In 1804, when Benjamin H. Latrobe (the designer of the United States Capitol) was living in New Castle, the architect performed a survey and designed a town plan along with his apprentices Robert Mills (later designer of the Washington Monument) and William Strickland (architect of the Second Bank of the United States).
The streets around the Court House were regraded so that it would loom above street level as a result of the town plan Latrobe devised.
As part of this survey, the architects drew detailed images of the New Castle Court House that were used for later restorations.
The oldest known courthouse on the site dates back to 1687. The building was originally both a courthouse and a seat of government, as the building was used by the colonial assembly. During the years when Pennsylvania and Delaware shared an assembly, it generally met in Philadelphia, but occasionally met in New Castle instead, such as in 1684 and 1690. In 1700, the Pennsylvania colonial assembly met in the recently constructed Court House, with William Penn in attendance. In addition to the routine passing of laws, Penn's primary focus during the 1700 session was healing a rift that had developed between the assemblymen from Pennsylvania proper and those from Delaware.
This attempt was unsuccessful. In 1704, after the Pennsylvania counties sought to expand their representation in the assembly, the three Delaware counties demanded, and were granted, an independent assembly so they could pass their own laws. They chose William Rodney, grandfather of Caesar Rodney, as their first Speaker. From that year on, the assembly that met in the Court House governed Delaware alone.
This earlier building was mostly destroyed by a fire in 1729 caused by a prisoner's attempt to escape. The Court House was rebuilt on the same site within a year, and the foundation of the original is still visible.
In 1774, the assembly met here and chose three of Delaware's most famous politicians to represent the colony in the Continental Congress: Thomas McKean, Caesar Rodney, and George Read. It was in this building that the assembly, at McKean's urging, voted on June 13, 1776 to sever all ties with England as well as the state of Pennsylvania, with which Delaware still shared a governor. The assembly resolved that:
Whereas it is become absolutely necessary for the safety, protection, and happiness of the good people of this colony, forthwith to establish some authority adequately to the exigency of their affairs, until a new government can be formed; and Whereas the representatives of the people, in this assembly met, alone can and ought at this time, to establish such temporary [authority]. Resolved unanimously, that all persons holding any office, civil or military, in this colony, on the 13 day of June instant, may and shall continue to execute the same, in the name of the
government of the Counties of New Castle, Kent, and Sussex Upon Delaware, as they used legally to exercise it in the name of the King, until a new government shall be formed, agreeable to the resolution of Congress of 15 May instant.
When the Congress issued the Declaration of Independence, it was read to the citizens of the town from the balcony on the second floor. Later in 1776, Delaware's first Constitution was drafted and adopted by a convention including Read, McKean, and Nicholas Van Dyke inside the Court House. This was the first adoption of a state constitution following the issuance of the Declaration. The building served as the state's first capitol until 1777, when Philadelphia and Wilmington were captured by the British. In light of the danger, Delaware's government moved south, establishing Dover as the new capital city. The building continued to house both state and federal judicial proceedings until 1888.
In 1804, when Benjamin H. Latrobe (the designer of the United States Capitol) was living in New Castle, the architect performed a survey and designed a town plan along with his apprentices Robert Mills (later designer of the Washington Monument) and William Strickland (architect of the Second Bank of the United States). The streets around the Court House were regraded so that it would loom above street level as a result of the town plan Latrobe devised. As part of this survey, the architects drew detailed images of the New Castle Court House that were used for later restorations.
New Castle Court House Museum