The size and composition of a marching band can vary greatly. Some bands have fewer than twenty members, and some have over 500.

A marching band is typically led by one or more drum majors, who are called field commanders in some ensembles. Other student leaders may include field lieutenants and captains of sections such as brass, drumline, and woodwinds. The drum major often conducts the band, sometimes using a large baton or mace. In many school bands, the drum major is the student leader of the band, followed by students within the band that lead a section, squad, letter, row, etc. Sometimes there is more than one drum major; one may be the head drum major who runs rehearsals and who stands on the 50-yard line while conducting, the other often directs from convenient angles (should the marching block not be facing forward) and/or functions as an apprentice of sorts. Bands may also be led by a more traditional conductor, especially during field shows, where a stationary conductor on a ladder or platform may be visible throughout the performance. Aural commands – such as vocal orders, clapping, or a whistle – may be used to issue commands as well.

American marching bands vary considerably in their instrumentation. Some bands omit some or all woodwinds, but it is not uncommon to see piccolos, flutes, clarinets, alto saxophones, and tenor saxophones. Bass clarinets, alto clarinets and baritone saxophones are more likely to be found in a high school marching band. Bassoons and oboes are very seldom found on a field, due to their "exotic" sound, risk of incidental damage, the impracticality of marching with an exposed double reed, and high sensitivity to weather.

The brass section usually includes trumpets or cornets, mellophones and/or alto horns (instead of French horns), B♭ tenor trombones, euphoniums or marching baritones, and tubas or sousaphones, a version of which also converted to be carried over the right shoulder and called a contra, sousaphones are named after John Phillip Sousa). Some upright tubas can also be converted, with a moveable lead pipe which allows the player to face the bell towards the box or crowd. These are normally carried over the shoulder and do not wrap around the player as the original Sousaphone did. E♭ soprano cornets are sometimes used to supplement or replace the high woodwinds. Some especially large bands use flugelhorns and bass trombones. The Saxophone, while not a true brass instrument, is often grouped with the brass section depending on their part. The Tenor and Bari-sax are more commonly placed in the Brass section than the Alto due to their low pitch and similarity in tone to the baritone horn.

Marching percussion (often referred to as the drumline, battery, or back battery) typically includes snare drums, tenor drums, bass drums, and cymbals and are responsible for keeping tempo for the band, which ultimately is the center snare's feet or the bottom bass drummer's beat. The drum major derives his/her title from his/her role in visually representing their tempo. All of these instruments have been adapted for mobile, outdoor use. Marching versions of the glockenspiel (orchestra bells), xylophone, and marimba are also rarely used by some ensembles. Historically, the percussion section also employed mounted timpani that featured manual controls.

For bands that include a front ensemble (also known as the pit or auxiliary), stationary instrumentation may include orchestral percussion such as timpani, tambourines, maracas, cowbells, congas, wood blocks, marimbas, xylophones, bongos, vibraphones, congas, timbales, claves, guiros, and chimes or tubular bells,concert bass drums, and gongs, as well as a multitude of auxiliary percussion equipment. Drum sets, purpose-built drum racks, and other mounted instruments are also placed here. Until the advent of the pit in the early 1980s, many of these instruments were actually carried on the field by marching percussionists by hand or on mounting brackets. Some bands also include electronic instruments such as synthesizers, electric guitars, and bass guitar, along with the requisite amplification. If double-reed or string instruments are used, they are usually placed here, but even this usage is very rare due to their relative fragility. Unusual percussive instruments are sometimes used, including brake drums, empty propane tanks, trashcans, railroad ties, stomping rigs, and other interesting sounds.