Born John Dean Cooper, but known to his friends as "Jeff", Cooper was a Marine Lieutenant Colonel who served in both World War II and the Korean War resigning his commission in 1956. He received a bachelor's degree in political science from Stanford University and, in the mid-1960s, a master's degree in history from the University of California, Riverside.

In 1976, Cooper founded the American Pistol Institute (API) in Paulden, Arizona (later the Gunsite Training Center). Cooper also began teaching shotgun and rifle classes to law enforcement and military personnel as well as civilians and did on-site training for individuals and groups around the world. He sold the firm in 1992 but continued living on the Paulden ranch. He was known for his advocacy of large caliber handguns, especially the Colt 1911 and the .45 ACP cartridge.

Jeff Cooper conceived and designed the Bren Ten pistol around the 10 mm Auto, based on the Czech CZ 75 design. The cartridge was more powerful than both the 9 mm Luger and the .45 ACP round.

His second major contribution to firearms design was what he termed a Scout Rifle. These bolt action carbines are typically .30 caliber (7.62 mm), less than 1 meter in length and less than 3 kilograms in weight, with iron and optical sights, and fitted with practical slings (such as Ching slings) for shooting and carrying and capable of hitting man-sized targets out to 450 meters without scopes. Typically they employ forward-mounted low-power long eye relief scopes or sights to afford easy access to the top of the rifle action for rapid reloading. Steyr, Ruger, Savage, and several other gun makers now manufacture Scout rifles that roughly match Cooper's specifications, but most lack auxiliary iron sights.

Cooper defined his goal: "... a general-purpose rifle is a conveniently portable, individually operated firearm, capable of striking a single decisive blow, on a live target of up to 200 kilos in weight, at any distance at which the operator can shoot with the precision necessary to place a shot in a vital area of the target."

Cooper died peacefully at his home on the afternoon of Monday, September 25, 2006.

Cooper's modern technique defines pragmatic use of the pistol for personal protection. The modern technique emphasizes two-handed shooting using the Weaver stance, replacing the once-prevalent one-handed shooting. The five elements of the modern technique are:

• A large caliber pistol, preferably a semi-auto
• The Weaver stance
• The presentation
• The flash sight picture
• The compressed surprise trigger break

Cooper favored the Colt M1911 and its variants. There are several conditions of readiness in which such a weapon can be carried. Cooper promulgated most of the following terms:

• Condition Four: Chamber empty, no magazine, hammer down.
• Condition Three: Chamber empty, full magazine in place, hammer down.
• Condition Two: A round chambered, full magazine in place, hammer down.
• Condition One: A round chambered, full magazine in place, hammer cocked, safety on.
• Condition Zero: A round chambered, full magazine in place, hammer cocked, safety off.

Some of these configurations are safer than others (for instance, a single action pistol without a firing pin safety should never be carried in Condition 2), while others are quicker to fire the gun (Condition 1). In the interest of consistent training, most agencies that issue the 1911 specify the condition in which it is to be carried as a matter of local doctrine.

This firearm condition system can also be used to refer to other firearm actions, particularly when illustrating the differences between carry modes considered to be safe for various actions. For example, DA/SA is designed to be carried in Condition 2, which is not safe for 1911s without firing pin safeties.

The most important means of surviving a lethal confrontation, according to Cooper, is neither the weapon nor the martial skills. The primary tool is the combat mindset, set forth in his book, Principles of Personal Defense. In the chapter on awareness, Cooper presents an adaptation of the Marine Corps system to differentiate states of readiness:

The color code, as originally introduced by Jeff Cooper, had nothing to do with tactical situations or alertness levels, but rather with one's state of mind. As taught by Cooper, it relates to the degree of peril you are willing to do something about and which allows you to move from one level of mindset to another to enable you to properly handle a given situation. Cooper did not claim to have invented anything in particular with the color code, but he was apparently the first to use it as an indication of mental state.

• White - Unaware and unprepared. If attacked in Condition White, the only thing that may save you is the inadequacy or ineptitude of your attacker. When confronted by something nasty, your reaction will probably be "Oh my God! This can't be happening to me."
• Yellow - Relaxed alert. No specific threat situation. Your mindset is that "today could be the day I may have to defend myself." You are simply aware that the world is a potentially unfriendly place and that you are prepared to defend yourself, if necessary. You use your eyes and ears, and realize that "I may have to SHOOT today." You don't have to be armed in this state, but if you are armed you should be in Condition Yellow. You should always be in Yellow whenever you are in unfamiliar surroundings or among people you don't know. You can remain in Yellow for long periods, as long as you are able to "Watch your six." (In aviation 12 o'clock refers to the direction in front of the aircraft's nose. Six o'clock is the blind spot behind the pilot.) In Yellow, you are "taking in" surrounding information in a relaxed but alert manner, like a continuous 360 degree radar sweep. As Cooper put it, "I might have to shoot."
• Orange - Specific alert. Something is not quite right and has gotten your attention. Your radar has picked up a specific alert. You shift your primary focus to determine if there is a threat (but you do not drop your six). Your mindset shifts to "I may have to shoot HIM today," focusing on the specific target which has caused the escalation in alert status. In Condition Orange, you set a mental trigger: "If that goblin does 'x', I will need to stop him." Your pistol usually remains holstered in this state. Staying in Orange can be a bit of a mental strain, but you can stay in it for as long as you need to. If the threat proves to be nothing, you shift back to Condition Yellow.
• Red - Condition Red is fight. Your mental trigger (established back in Condition Orange) has been tripped. If "X" happens I will shoot that person.

The USMC also uses "Condition Black" as actively engaged in combat, as do some of Cooper's successors, but Cooper always felt this was an unnecessary step and not in keeping with the mindset definition of the color code since it is a state of action.

In short, the Color Code helps you "think" in a fight. As the level of danger increases, your willingness to take certain actions increases. If you ever do go to Condition Red, the decision to use lethal force has already been made (your "mental trigger" has been tripped).

The following are some of Cooper's additional comments on the subject.

"Considering the principles of personal defense, we have long since come up with the Color Code. This has met with surprising success in debriefings throughout the world. The Color Code, as we preach it, runs white, yellow, orange, and red, and is a means of setting one’s mind into the proper condition when exercising lethal violence, and is not as easy as I had thought at first.

There is a problem in that some students insist upon confusing the appropriate color with the amount of danger evident in the situation. As I have long taught, you are not in any color state because of the specific amount of danger you may be in, but rather in a mental state which enables you to take a difficult psychological step."Now, however, the government has gone into this and is handing out color codes nationwide based upon the apparent nature of a peril. It has always been difficult to teach the Gunsite Color Code, and now it is more so.

We cannot say that the government’s ideas about colors are wrong, but that they are different from what we have long taught here."The problem is this: your combat mind-set is not dictated by the amount of danger to which you are exposed at the time. Your combat mind-set is properly dictated by the state of mind you think appropriate to the situation. You may be in deadly danger at all times, regardless of what the Defense Department tells you. The color code which influences you does depend upon the willingness you have to jump a psychological barrier against taking irrevocable action. That decision is less hard to make since the jihadis have already made it."

He further simplified things in Vol 13 #7 of his Commentaries.

"In White you are unprepared and unready to take lethal action. If you are attacked in White you will probably die unless your adversary is totally inept.

In Yellow you bring yourself to the understanding that your life may be in danger and that you may have to do something about it.

In Orange you have determined upon a specific adversary and are prepared to take action which may result in his death, but you are not in a lethal mode.

In Red you are in a lethal mode and will shoot if circumstances warrant."

Cooper advocated four basic rules of gun safety:

1. All guns are always loaded. Even if they are not, treat them as if they are.
2. Never let the muzzle cover anything you are not willing to destroy. (For those who insist that this particular gun is unloaded, see Rule 1.)
3. Keep your finger off the trigger till your sights are on the target. This is the Golden Rule. Its violation is directly responsible for about 60 percent of inadvertent discharges.
4. Identify your target, and what is behind it. Never shoot at anything that you have not positively identified.

Cooper is best known for his revolutionary work in pistol training, but he favored the rifle for tactical shooting. He often described the handgun as a convenient-to-carry stopgap weapon, allowing someone the opportunity to get to a rifle.

"Personal weapons are what raised mankind out of the mud, and the rifle is the queen of personal weapons."

"The rifle is a weapon. Let there be no mistake about that. It is a tool of power, and thus dependent completely upon the moral stature of its user. It is equally useful in securing meat for the table, destroying group enemies on the battlefield, and resisting tyranny. In fact, it is the only means of resisting tyranny, since a citizenry armed with rifles simply cannot be tyrannized."

"The rifle itself has no moral stature, since it has no will of its own. Naturally, it may be used by evil men for evil purposes, but there are more good men than evil, and while the latter cannot be persuaded to the path of righteousness by propaganda, they can certainly be corrected by good men with rifles."

...Jeff Cooper, The Art of the Rifle

In the early 1980s, Cooper published an article describing his ideal of a general-purpose rifle, which he dubbed a Scout Rifle. In late 1997, Steyr-Mannlicher produced a rifle to his "Scout" specifications, with Cooper's oversight during the engineering and manufacturing process. While not a spectacular sales success, these rifles nevertheless sold quite well and are still being produced. Cooper considered the Steyr Scout "perfect" and often made the point that "I've got mine!" Riflemen regard Cooper's development of the Scout Rifle concept, and his subsequent work on the evolution of the Steyr-Mannlicher Scout rifle, as his most significant and enduring contributions to riflecraft.

In the 1960s he coined the term hoplophobia, an irrational fear of weapons.

In addition to his books on firearms and self defense, Cooper wrote several books recounting his life adventures plus essays and short stories, including Fire Works (1980), Another Country: Personal Adventures of the Twentieth Century (1992); To Ride, Shoot Straight and Speak the Truth (1998); and C Stories (2004). His daughter Lindy Wisdom published a biography, Jeff Cooper: the Soul and the Spirit (1996).

Cooper was also regarded as one of the world's foremost authorities on big game hunting.

Some of the comments from his "Gunsite Gossip" newsletter were printed in gun magazines as "Cooper's Corner" and later were compiled into The Gargantuan Gunsite Gossip. These were his thoughts on firearms interleavened with his wide-ranging musings on many other subjects, and acquired a large US and international following from the 1980s up to his death.

A complete bibliography of Jeff Cooper's writings from 1947 onwards is available at the Jeff Cooper Bibliography Project.

Cooper was the Founding President and Honorary Lifetime Chairman of the International Practical Shooting Confederation. However, he was critical of the way the IPSC departed from the original focus on practical weapons toward what he called rooney guns -- highly-modified pistols which were not appropriate for practical daily service, and which were tuned to fire only ammunition with small propellant charges. Numerous of his "Gunsite Gossip / Cooper's Corner" articles dealt with this issue and the "gamesmen" who had caused the IPSC to deteriorate.