In the 1950s, when Elvis Presley was leading the rock-and-roll revolution, the McGuire Sisters were genteel holdovers from an earlier time.

In their proper, unthreatening way, Christine, Dorothy and Phyllis McGuire had a popularity rivaling that of Elvis himself. They were on countless magazine covers and TV shows, appeared in nightclubs and concert halls, and had 10 songs in the Billboard Top 20.

Their two No. 1 hits — “Sincerely” and “Sugartime” — reflected the trio’s sweet, earnest image. The sisters, who began singing in church in Ohio during the 1930s, had an uncanny sense of timing and close harmony matched by a perky, ever-smiling stage manner.

They were so close that they sometimes held hands as they sang or took their bows. Yet the spotlight seemed to shine the brightest on Phyllis McGuire, the youngest sister, who always stood in the center and sang the lead.

Phyllis McGuire and Chicago mob boss Sam Giancana in a London nightclub in 1962.

Phyllis McGuire and Chicago mob boss Sam Giancana in a London nightclub in 1962. (Associated Press)

Ms. McGuire, who was 89 and the last surviving McGuire sister, died Dec. 29 at her home in Las Vegas. Her death was announced in a paid notice in the Las Vegas Sun newspaper. The cause was not disclosed.

Even as musical tastes began to change, the McGuire Sisters kept going strong. By 1960, each of the sisters was earning more than $1 million a year.

After a final appearance on “The Ed Sullivan Show” in 1968, they parted ways. Christine and Dorothy were married and raising families. Phyllis, who had been married once in the 1950s, was single and raising eyebrows.

Rumors began circulating, and then were confirmed without apology by Ms. McGuire, that she was the girlfriend of Chicago mob boss Sam Giancana.

They had met in 1959, when Ms. McGuire and her sisters were performing at the Desert Inn, one of the Las Vegas casinos run by Giancana. “Who’s the one in the middle?” he reportedly asked.

Ms. McGuire, who had a weakness for the blackjack tables, ran up a debt of tens of thousands of dollars at the Desert Inn. Giancana, watching from afar, told his casino boss to “eat it” — or forgive the debt.

Thus began one of the most unlikely romances in show business. Giancana, who got his start as Al Capone’s driver in Chicago, was widowed, bald and in his 50s. He had been arrested dozens of times, linked to crimes from illegal gambling to murder, and had served time in prison.

Ms. McGuire was still in her 20s and had a public image as benign and carefully arranged as one of the McGuire Sisters’ hit songs. Giancana sent her lavish gifts of jewelry and furs and often met her overseas, wherever the sisters were performing. Strange as it may seem, everyone who knew them agreed they were in love.

“It’s amazing that it ever took place,” William F. Roemer Jr., an FBI agent who tracked Giancana for years, told the Los Angeles Times in 1995. “She had everything. She had beauty. She had money. Yet, she fell in love with this gangster. I could never figure it out.”

In 1961, FBI agents wiretapped their room in a Phoenix motel. Later, after being questioned about Giancana’s activities, Ms. McGuire pleaded ignorance. Federal authorities asked her to cooperate, with the implicit threat that her career would be ruined if her affair with a mafia kingpin were exposed.

“She said she would, but she never did,” Roemer said. “She never cooperated with us. She double-crossed us really.”

In 1965, Ms. McGuire testified before a grand jury investigating Giancana for racketeering. She admitted that they had a relationship and that she was aware of his reputation but maintained she knew nothing about his life of crime.

The revelation “really hurt our career,” Ms. McGuire told the Chicago Tribune in 1989. “We were blacklisted for a while on TV. . . . We were America’s sweethearts, and for one of America’s sweethearts to be with that man . . . ”

Giancana went to prison for a year in 1965, then lived in Mexico and South America, where he was visited by Ms. McGuire. He later moved back to suburban Chicago and was cooking in his basement in 1975 when an assailant entered and shot him seven times in the head. The murder was never solved.

“I just knew that I liked the man,” Ms. McGuire told the Los Angeles Times in 1986. “He was very nice to me. And if he had done all those things they said he did, I wondered why in God’s name he was on the street and not in jail.”

Phyllis Jean McGuire was born Feb. 14, 1931, in Middletown, Ohio, and grew up in the nearby town of Miamisburg.

Her father was a steelworker, and her mother was a minister. Phyllis was 4 when she and her sisters began singing in their mother’s church. (Christine was five years older than Phyllis, Dorothy three years older.) Before long, they were performing at weddings, revival meetings and the USO. They had a long engagement at a hotel in Dayton, Ohio, and appeared on radio and television.

In 1952, the McGuire Sisters moved to New York and landed an eight-week engagement on Kate Smith’s radio show. They later won a talent contest and were featured on Arthur Godfrey’s popular TV show. Their first Top 10 hits came in 1954, with “Goodnight, Sweetheart, Goodnight” and “Muskrat Ramble,” and “Sincerely” reached No. 1 in 1955.

Two years later, they recorded “Sugartime,” by Charlie Phillips and Odis Echols, which climbed to No. 1 in 1958 and became the sisters’ signature tune:

Sugar in the mornin’

Sugar in the evenin’

Sugar at suppertime

Be my little sugar

And love me all the time

Even before the sisters broke up in 1968, Phyllis McGuire began working on her own, including an acting role in the 1963 Frank Sinatra film “Come Blow Your Horn.”

By 1985, the McGuire Sisters were ready to launch a comeback, but they struggled to re-create the instinctive harmonies they had in their youth.

“We rehearsed eight hours a day, five days a week for six months,” Phyllis McGuire told the Tribune in 1989. “Then one day, after perspiring and toiling and worrying, we started rehearsing and all in the same instance we looked at each other and said, ‘My God, thank you, that’s it.’ We had it back.”

Wearing matching dresses and hairstyles, the sisters performed in nightclubs and concert venues until 2004. Dorothy McGuire died in 2012, Christine McGuire in 2018.

Phyllis McGuire’s early marriage to Neal Van Ells ended in divorce. After Giancana’s death, she was occasionally linked to wealthy men, but she never remarried and had no immediate survivors.

A 1995 HBO film, “Sugartime,” starring Mary-Louise Parker and John Turturro, portrayed Ms. McGuire’s life with Giancana. She denounced it as “riddled with blatant inaccuracies, exaggerations and distortions.”

In 1999, after Las Vegas police stopped her limousine and questioned her driver, the 68-year-old Ms. McGuire emerged from the car “screaming, waving and flailing her arms” and was arrested for head-butting and kicking a police officer. Charges were dropped after a plea deal.

Ms. McGuire was an astute investor, and it is widely believed that much of Giancana’s fortune came into her hands. She had a jewelry collection said to rival those of Elizabeth Taylor and Philippine first lady Imelda Marcos.

She lived on one of the grandest estates in Las Vegas, in a house that contained, under its roof, a 40-foot replica of the Eiffel Tower and another of the Arc de Triomphe. Steel shutters could cover the bulletproof windows with the touch of a button. She had five gardeners and a pond with black swans floating by.

“I’m not ashamed of my past,” she told Vanity Fair, describing everything from music to the mob. “I was doing what I honestly felt.”

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