Quick answer: early cheerleaders wore everyday college clothing - lettered sweaters, and for women, ankle-length wool skirts. Through the 20th century uniforms became shorter, lighter and more athletic: pleated knee-length skirts in the 1960s, slimmer striped sweaters in the 1970s, and today's fitted polyester-spandex competition uniforms, often in shiny metallic fabrics.
Early Uniforms (Early 1900s-1940s)
When cheerleading was a mostly male activity, yell leaders wore smart college clothing. At Ohio State, for example, the 1930s "Gentlemen of the Cheer" performed in jackets, sweaters and ties. As women joined, the typical uniform became a heavy sweater - often a long, buttoned cardigan - with the school's letter sewn on, worn over a collared shirt or turtleneck, with a very modest ankle-length wool skirt. Lettering was traditionally chenille (a fluffy, tufted fabric), later replaced by the flatter tackle twill used on sports kit.
Mid-Century (1950s-1960s)
By the 1950s, women's skirts were still below the knee, worn with sweaters, white socks and saddle shoes. In the 1960s, uniforms changed quickly: long wool skirts gave way to knee-length pleated cotton skirts, and short-sleeved crew-neck sweaters replaced long cardigans. At Ohio State, skirts were shortened to roughly their current length in 1968 - prompting letters to the university about "proper feminine attire."
This was also when cheer uniforms became a business. Jean Lee Originals of Goshen, Indiana, became the first company to market cheerleading uniforms widely in the 1960s and 1970s, making custom-fitted uniforms; its owner, Jean Marie Harter, is credited with designing the pleated skirt and double-stripe sweater styles still seen today.
The 1970s and 1980s
Uniforms became lighter and more athletic: slimmer, shorter sweaters with striped fronts, and new companies making uniforms in standard sizes rather than custom fits. Fabrics moved from all-wool towards synthetics such as double-knit polyester, which made brighter colours easier and uniforms easier to care for. From the mid-1980s to mid-1990s, turtlenecks worn under sweaters, sweatshirts or vests were popular, with mid-thigh skirts trimmed with coloured stripes.
The Competition Era
As competitive and All Star cheer grew, uniforms were redesigned around stunting and tumbling. Most uniforms today are made from polyester-spandex blends that stretch with the body. Competitive teams often use shiny metallic "foil" fabrics, rhinestones and printed (dye-sublimated) designs that stand out under arena lights. Uniforms are worn with matching briefs - known by names like "spankies" or "lollies" - and many All Star uniforms use a fitted top rather than a sweater-style shell.
Rules Shape Uniforms Too
Modern uniforms are also shaped by competition rules. The USASF All Star rules, for example, set minimums for skirt and shorts length, require tops to be secured over at least one shoulder or around the neck, and only allow exposed midriffs for older Elite-tier age groups. See What Is a Cheer Competition Uniform?.
A Note on Dates
Exact dates for style changes varied from school to school, and the examples above - such as Ohio State's 1968 change - show when changes happened at particular places, not everywhere at once. The sources we used are listed below.
Sources: Cheerleading uniform (Wikipedia); Of Pompons and Human Pyramids (Ohio State University Libraries); More comfortable materials give cheerleaders reason to cheer (TribLive)