LOS ANGELES — Carol Spencer, 86, may be the most influential fashion designer you’ve never heard of.

In the mid-1960s, she made a red pencil skirt with a white sleeveless blouse that had red stitching and three red buttons down the front. Short white gloves came with it. Thousands sold.

In the 1970s, well aware that the counterculture’s loosening dress code and mores had made it to the mainstream, Ms. Spencer designed a red bandanna halter maxi-dress and a matching leisure shirt for men. Those designs were popular, too.

In the Nancy Reagan 1980s, Ms. Spencer aimed for high-end appeal, making a one-shouldered ball gown in blue jacquard with an organza flower at the nipped-in waist and a cape. One of Mrs. Reagan’s go-to couturiers personally approved the gown to be sold under his name: “Oscar de la Renta for Barbie.”