Born at the Grand Hotel National in Lucerne, Switzerland, as Maria Mercedes Morgan, she was a daughter of Henry Hays Morgan, Sr. (
1860–1933), an American diplomat, who served as U.S. consul general in Buenos Aires, Argentina; Berlin, Germany; Amsterdam, Netherlands; Havana, Cuba; and Brussels, Belgium.
Her mother was his second wife, the former Laura Delphine Kilpatrick (1877–1956); the couple was married in 1894 and divorced in 1927.

Her maternal grandfather, Hugh Judson Kilpatrick (1836–1881), was a Union Army general during the American Civil War, who also served as the U.S. minister to Chile.
Her maternal grandmother, Luisa Kilpatrick, née Valdivieso Araoz, was a member of a wealthy Chilean family that had emigrated from Spain in the 17th century.

Morgan, who adopted the name Gloria as a teenager, had five siblings:
Laura Consuelo Morgan (17 December 1901 – 26 August 1979), aka Tamar.
She married Count Jean de Maupas du Juglart, Ambassador Benjamin Thaw Jr., and Alfons B. Landa (née Alfonso Beaumont Howard Landa).

Thelma Morgan (1904–1970), her identical twin.
She became a mistress of Edward, Prince of Wales and married James Vail Converse and Marmaduke Furness, 1st Viscount Furness.

Harry Hays Morgan Jr. (1898–1983), who became a movie actor (stage names Harry Hayes Morgan and Harry Hays Morgan).
He was married to Ivor Elizabeth O'Connor, Edith Churchill Gordon, and Sybil Robertina "Robin" Boyce Willys.

Constance Morgan (1887–1892), a half sister, a child of her father's first marriage to Mary E. Edgerton.

Gladys Morgan (14 September 1889–15 August 1958), another half sister from her father's first marriage; she was known as Margaret and married J. Henderson.

Gloria Morgan was educated by governesses and in convents in Europe as well as New York City, where she attended the Catholic Convent of the Sacred Heart (in the Manhattanville section of the city), the Skerton Finishing School, and Miss Nightingale's School.
In October 1921, with their father's permission, Morgan and her sister Thelma, both reportedly 16 years of age, ended their schooling and moved by themselves into an apartment at 40 Fifth Avenue, a private townhouse.
The sisters had some minor roles in silent movies, using the names Gloria and Thelma Rochelle.
Their debuts were as extras in the 1922 Marion Davies vehicle The Young Diana.

Known as "The Magnificent Morgans", Gloria and Thelma Morgan were popular society fixtures, even as teenagers.
British photographer Cecil Beaton described them as "alike as two magnolias, and with their marble complexions, raven tresses, and flowing dresses, with their slight lisps and foreign accents, they diffuse a Ouida atmosphere of hothouse elegance and lacy femininity. ...
Their noses are like begonias, with full-blown nostrils, their lips richly carved, and they should have been painted by Sargent, with arrogant heads and affected hands, in white satin with a bowl of white peonies near by."
Born at the Grand Hotel National in Lucerne, Switzerland, as Maria Mercedes Morgan, she was a daughter of Henry Hays Morgan, Sr. (1860–1933), an American diplomat, who served as U.S. consul general in Buenos Aires, Argentina; Berlin, Germany; Amsterdam, Netherlands; Havana, Cuba; and Brussels, Belgium. Her mother was his second wife, the former Laura Delphine Kilpatrick (1877–1956); the couple was married in 1894 and divorced in 1927.
Her maternal grandfather, Hugh Judson Kilpatrick (1836–1881), was a Union Army general during the American Civil War, who also served as the U.S. minister to Chile. Her maternal grandmother, Luisa Kilpatrick, née Valdivieso Araoz, was a member of a wealthy Chilean family that had emigrated from Spain in the 17th century.
Morgan, who adopted the name Gloria as a teenager, had five siblings:
Laura Consuelo Morgan (17 December 1901 – 26 August 1979), aka Tamar. She married Count Jean de Maupas du Juglart, Ambassador Benjamin Thaw Jr., and Alfons B. Landa (née Alfonso Beaumont Howard Landa).
Thelma Morgan (1904–1970), her identical twin. She became a mistress of Edward, Prince of Wales and married James Vail Converse and Marmaduke Furness, 1st Viscount Furness.
Harry Hays Morgan Jr. (1898–1983), who became a movie actor (stage names Harry Hayes Morgan and Harry Hays Morgan). He was married to Ivor Elizabeth O'Connor, Edith Churchill Gordon, and Sybil Robertina "Robin" Boyce Willys.
Constance Morgan (1887–1892), a half sister, a child of her father's first marriage to Mary E. Edgerton.
Gladys Morgan (14 September 1889–15 August 1958), another half sister from her father's first marriage; she was known as Margaret and married J. Henderson.
Gloria Morgan was educated by governesses and in convents in Europe as well as New York City, where she attended the Catholic Convent of the Sacred Heart (in the Manhattanville section of the city), the Skerton Finishing School, and Miss Nightingale's School. In October 1921, with their father's permission, Morgan and her sister Thelma, both reportedly 16 years of age, ended their schooling and moved by themselves into an apartment at 40 Fifth Avenue, a private townhouse. The sisters had some minor roles in silent movies, using the names Gloria and Thelma Rochelle. Their debuts were as extras in the 1922 Marion Davies vehicle The Young Diana.
Known as "The Magnificent Morgans", Gloria and Thelma Morgan were popular society fixtures, even as teenagers. British photographer Cecil Beaton described them as "alike as two magnolias, and with their marble complexions, raven tresses, and flowing dresses, with their slight lisps and foreign accents, they diffuse a Ouida atmosphere of hothouse elegance and lacy femininity. ... Their noses are like begonias, with full-blown nostrils, their lips richly carved, and they should have been painted by Sargent, with arrogant heads and affected hands, in white satin with a bowl of white peonies near by."
Gloria Morgan Vanderbilt