She was a figure out of fiction, right down to her Jane Austen name. The daughter of a baroness and a British major (a spy during World War II), Marianne Faithfull — who died this week at 78 — was discovered by the Rolling Stones’ manager, Andrew Loog Oldham, at a record release party in the 1960s while still in her teens. “My first move was to get a Rolling Stone as a boyfriend,” she was often quoted as having said. “I slept with three and decided the lead singer was the best bet.”

The bet paid off for both parties. Mick Jagger and Ms. Faithfull dated from 1966-70 and during that time she recorded a series of pop songs, most memorably “As Tears Go By.” Mr. Jagger wrote imperishable Stones hits like “Wild Horses” under the direct inspiration of Ms. Faithfull — lovely, feckless, druggie and unfettered. She was “a wonderful friend,” Mr. Jagger wrote on Instagram this week, “a beautiful singer and a great actress.”

She was also a style paragon from the outset.

“She seemed to touch all the moments, from Mod to rich hippie to bad girl and punk, corsets to leather to the nun outfit she wore when she performed with Bowie,” the designer Anna Sui said this week by phone. “She was there, through all those periods — performing, participating in events, acting and singing and also in the tabloids, very much in the eyes of anybody loving those periods.”

A British journalist once described Ms. Faithfull, in the late 1960s, as “the flowing-haired, miniskirted, convention-knocking epitome” of a “drug generation” that her elders were challenged to understand. What more accurately she epitomized was a spirit of bohemian laissez-faire better located in class than any particular era.

Cultured, if not conventionally educated, Ms. Faithfull was as offhand about her looks as only a natural beauty could afford to be. And she was as indifferent to the straight-jacketing conventions of the bourgeoisie as those of her background (she spent her early years in an upscale commune her father founded in Oxfordshire) often are.