While the specifics of the title may differ between the United States Army, Marine Corps, and Air Force, all first sergeants can be identified by the presence of a lozenge-shaped (colloquially "diamond") figure on their rank insignia.

Previously, under the tables of organization approved by the Continental Congress in 1776 and 1779, there were four and three sergeants, respectively, authorized in each company.

[2] After the 1781 Battle of Green Spring, Ebenezer Denny called the company first sergeant "the most important officer.

A first sergeant may place a soldier under arrest and on restriction to quarters in certain cases, as well as manage all of the daily responsibilities of running the company/unit.

Historically, Army National Guard Soldiers, who often remained in uniform under Title 32, USC state status, until days or weeks shy of their 62nd birthday, often stayed in one unit for decades, especially with cases of senior enlisted promotions or appointments.

After the mass mobilization of Guard troops in the Persian Gulf War in 1991, many states began a statewide promotion system, resulting in Soldiers transferring units frequently.

A first sergeant has command leadership responsibilities and serves as the senior enlisted adviser to the commander at the company, battery or detachment level, while master sergeants have technical responsibilities within their respective occupational fields, and serve important leadership roles within various company or battery sections.

Ultimately, those selected for either rank are appointed based on suitability, previous duty assignments, and the needs of the Marine Corps.

[7] In the United States Air Force, first sergeants are special duty temporary ranks[8] and positional billets.

While both services don't have separate first sergeant ranks, those who hold these positional billets are senior to their non-diamond counterparts.

Some U.S. law enforcement agencies, especially state police and highway patrol organizations, have first sergeants, who are typically in charge or command of a detachment, district, region, area, barracks, or post consisting of anywhere from ten to fifty or more troopers or officers.

Most law enforcement first sergeants are mid-level management leaders, with ten to thirty or more years of service.

The NC Highway Patrol first sergeants for example, must complete the 6-weeks advanced police management training institute at the Southern Police Institute (SPI) in Louisville, KY. Other states also use SPI, or Northwestern University IL or the FBI National Academy (FBINA) Quantico, VA.

Some such state agencies may have a first sergeant in charge of special state police or highway patrol units such as SWAT, K-9, aviation, personnel, major traffic accident reconstruction, research, public information, logistics, training, recruitment, internal affairs, accreditation, inspections, mounted, motorcycle, communications, detectives, administration, and other specialized sections or services other than general patrol.