As of the 2020 census, the borough population was 20,118;[4] including suburbs in the neighboring townships, 37,695 live in the Carlisle urban cluster.

Carlisle is the smaller principal city of the Harrisburg–Carlisle metropolitan statistical area, which includes Cumberland and Dauphin and Perry counties in South Central Pennsylvania.

The U.S. Army War College, located at Carlisle Barracks, prepares high-level military personnel and civilians for strategic leadership responsibilities.

Carlisle Barracks is home of the United States Army Heritage and Education Center, an archives and museum complex open to the public.

The French-born fur trader James Le Tort may have built a cabin in the area as early as 1720.

American engineer John Armstrong Sr., a surveyor for the Penn family, laid the plan for the settlement of Carlisle in 1751.

[7] As a result of conflicts on the frontier with Native American tribes, a stockade was constructed in the settlement to protect against potential attacks in 1753.

The Pennsylvania guide, compiled by the Writers' Program of the WPA in 1940, described the early history of Carlisle's public square and the physical changes that had occurred by the first half of the 20th century, noting that the square, located at the[9]intersection of Hanover and High Sts., is now hardly recognizable as such, for the market house, courthouse, and church have encroached on it.

The square was the camping ground of Indian delegations in the tense days when the French were invading the Ohio Valley, the gathering place of Revolutionary mass meetings, and the nucleus of a compact little settlement.The settlement of Carlisle was largely supportive of the Patriot cause during the American Revolution, and numerous individuals from the settlement served in the Revolutionary War.

The church was also where Pennsylvania settlers met on July 12, 1774, to sign a document protesting the Boston Port Act.

[13] No longer standing but marked by a historical marker is the home of Ephraim Blaine, Commissary General of Revolutionary Army.

[14] Also, no longer standing but commemorated, is the home of Gen. John Armstrong Sr., "Hero of Kittanning," Revolutionary officer, and member of the Continental Congress.

Still standing is the gun shop of Thomas Butler Sr., an Irish immigrant, who manufactured long rifles during the French and Indian War.

Revolutionary War legend Molly Pitcher died in the borough in 1832, and her body lies buried in the Old Public Graveyard.

A decade later, during the Whiskey Rebellion in 1794, the troops of Pennsylvania and New Jersey assembled in Carlisle under the leadership of President George Washington.

Benjamin Rush, a signer of the Declaration of Independence, developed Carlisle Grammar School in 1773 and chartered it as Dickinson College—the first new college founded in the newly recognized United States.

During the war, an army of the Confederate States of America, under General Fitzhugh Lee, attacked and shelled the borough during the Battle of Carlisle on July 1, 1863, as part of the Gettysburg Campaign.

The United States government maintained the school, housed at Carlisle Barracks as an experiment in educating Native Americans and teaching them to reject tribal culture and to adapt to white society.

Athletic hero Jim Thorpe entered the school in 1907 and joined its football team under coach Glenn "Pop" Warner in 1908.

Playing halfback, Jim Thorpe led the team to startling upset victories over powerhouses Harvard, Army, and the University of Pennsylvania in 1911–12, bringing nationwide attention to the school.

In 2013, Apple opened an AppleCare device repair facility southeast of the Interstate 81 overpass over the Pennsylvania Turnpike to cover American customers east of the Mississippi River.

Partly because of its location at the intersection of two major trucking routes (I-81 and I-76), air pollution within the borough often falls within the range considered by the United States Environmental Protection Agency as "Unhealthy for Sensitive Groups" [i.e., children, the elderly, and people with respiratory or heart disease].