It was an epiphany at the Great American Music Hall in San Francisco that changed the course of Wesla Whitfield’s life.

“Some friends of mine took me to hear this singer Sarah Vaughan, and that was that,” Whitfield told me some two decades ago, in an extended interview for a documentary. “I’ve been trying ever since to get all the way from opera to jazz. I’ll never make it, but it’s a wonderful time trying.”

Whitfield, who died Friday night at the age of 70 in the Saint Helena home she shared with her husband, pianist Mike Greensill, left opera far behind. Over the course of four decades, 20 albums and hundreds of spellbinding performances, she played a key role in making San Francisco the most vital outpost for American Songbook exploration after New York City.

While some pigeonholed her as a cabaret singer, Whitfield’s artistry made distinctions between jazz and cabaret moot. An expert spelunker into the vast treasure trove of Broadway show tunes, Hollywood themes, and Tin Pan Alley songs churned out before the rock ‘n’ roll revolution, she turned standards into riveting tales marked by her fierce intelligence and emotional acuity.

“I’m trying to become a jazz singer, in the sense that I don’t scat, but then neither did Billie Holiday,” Whitfield said. “I tell the story of the song, and in jazz you can play with the notes a little. You can play with the rhythm, and you can play with a lot of stuff that helps tell the story. So I’m a storyteller, and I’m an apprentice jazz singer.”

The cabaret tag stuck partly because Whitfield became a regular presence in the country’s top cabaret venues rather than jazz clubs. She won a devoted following with long runs at the Algonquin Hotel’s storied Oak Room, the Hollywood Roosevelt Hotel’s Cinegrill, and her hometown spot, the York Hotel’s lamented Plush Room. But Whitfield surrounded herself with jazz players, most importantly Greensill, an ebulliently swinging Englishman and ace accompanist with a gift for crafting bespoke, harmonically insightful arrangements beautifully tailored for Whitfield’s voice.

Part of what made Whitfield such a powerful performer was her ability to set the stage for a song with a few pithy remarks, like the time she recast “You Don’t Know What Love Is” from torch song lament to passive-aggressive ode by mentioning that a former lover once sent her a recording of the song. No one has better summed up the American Songbook chapter devoted to masochistic love songs than when she once introduced Billie Holiday’s “Don’t Explain” as belonging to the “Hello! I’m a Doormat, Step on Me” genre.

Born Weslia Edwards in Santa Maria in 1947 when the Santa Barbara County town depended upon agriculture, Whitfield decided early on that she would be a singer. Around the age of five, she saw the singer Molly Bee on television, “and I thought that’s what I want to do,” she recalled. “I didn’t tell anybody because I was a little girl and they would say, ‘Don’t be silly. What do you know?’ But I knew I was going to be a singer.”

She loved showtunes, but vocal training in the 1960s focused on the European classical repertoire. After several years studying in Los Angeles she eventually made her way to the Bay Area and earned a B.A. in music from San Francisco State in 1972 (three decades later the school named her alumna of the year).

While singing in the San Francisco Opera chorus, she led a double life, engaging in “the lowest form of singing known to man,” she said. “I started as a singing cocktail waitress, part of a little group of singing cocktail waiters and waitresses, and three times a night we would all get up and do our songs and that would be our set. Slowly I built up a little following.”

Then came Sarah Vaughan, and her jazz epiphany. She had just started playing little gigs around the city in 1977 when two young teenage boys tried to mug her. As she walked away, one of them pulled out a gun and shot her in the back. The shooting left her without the use of her legs, a fact that she downplayed on stage until recent years when she started performing in her wheelchair.

A key turning point in her career came when she began collaborating with Greensill, who had spent four years playing in Hong Kong before settling in San Francisco in 1977. Whitfield’s pianist at the time liked to take long breaks, and Greensill ended up sitting in for several numbers one night. “He played really well, and talking to him afterwards I found out he also did arranging, so I said come by and we’ll talk about some tunes,” Whitfield recalled.

They were both married at the time, but over the course of five years performing together they each divorced and forged a close friendship, “then one night we looked at each other and whoops!” Whitfield said.

Married in 1985, they knew that Whitfield had to start recording to land better gigs. Her 1986 debut album “Just For a Thrill” featured tenor sax great Al Cohn on one of his last sessions and earned rave reviews from longtime San Francisco Examiner critic Phil Elwood.

She went on to make a series of exceptional recordings, often working with legendary jazz producer Orrin Keepnews. A particular standout is 2003’s September Songs (HighNote), a program of music by Alec Wilder, Kurt Weill, and Harry Warren featuring Kronos Quartet, pianist Tommy Flanagan, and drummer Tootie Heath among the special guests.

Throughout the years, Whitfield continued to measure herself by her musical heroes, “Carmen McRae and Anita O’Day and other people just unbelievably hip,” she said. “My husband is that hip and the musicians that work with me are all real jazz performers, so I’m trying. I figure I have 30 more years before I have to stop singing, or maybe not. But I’ll always be trying to be a little less square.”

Better than being hip, Wesla trafficked in truth, revealing the stark emotions and sometimes sublime beauty waiting to be uncovered in the American Songbook that she loved so dearly.