By the time she was 12, she was selected out of thousands of schoolchildren to join the children's chorus of New York's Metropolitan Opera. She won a singing contest at the age of 15 (wearing a dress made by her mother) which launched her career as a soloist. She debuted at La Conga club singing with a big band led by Desi Arnaz.
In 1939, she represented the Dominican Republic in that country's pavilion in the World's Fair that took place in New York. At the Fair, she sang, dressed in typical Dominican clothing, introducing merengue to the American audience for the first time. A student of many folklores of the West Indies, she also introduced calypso to American audiences. By the mid-1950s, both dance styles were among the most popular in the U.S.
After the World's Fair, she became popular as a nightclub club singer in the 1940s and 1950s. She was able to sing in seven languages. It was around this time she adopted the name Mónica Boyar (her producers thought her birth name, Argentina Mercedes, was too complicated to say for American audiences). She performed in some the ritziest hotspots around the world, including the Waldorf=Astoria Hotel in New York, the Lido Cabaret in Paris, and the Tropicana in Havana, among others. She performed in front of royal figures such as Prince Rainier of Monaco, Queen Beatrix of the Netherlands and Queen Elizabeth II of the United Kingdom.
She was briefly married first to Canadian businessman Steve McDonald in the 1940s; the marriage was annulled. She then wed fellow Dominican Federico Horacio Henríquez Vásquez from 1943-1947. Henríquez was the nephew of Horacio Vásquez, the last elected president of the Dominican Republic before the rise of dictator Rafael Leonidas Trujillo. Boyar was widowed when her husband was killed by Trujillo's forces. She then married Canadian actor Leslie Nielsen from 1950-1956. He later became famous for his roles in the Airplane and Naked Gun franchises. She was also briefly married to comedian Lee Tully in 1958. She was called the "Satin Latin Song Stylist". Famed journalist Walter Winchell referred to her as the finest Latin talent in the entertainment field in 1960.
Her godfather was Juan Rafael Estrella Ureña, the last acting president of the Dominican Republic before Trujillo took over the nation (Estrella was briefly Vice-President under Trujillo). Due to the contentious relationship between Estrella and Trujillo, she was unable to set foot on the island of her birth for over three decades. She became an anti-Trujillo proponent, even recording two albums of self-penned work titled Marcha a Santo Domingo and Capita fue a la guerra. As a result, she is more well known in the United States than her native Dominican Republic.
A former art student, she later studied painting and fashion design and became known for her lines of hats and purses, where she designed for some of the biggest names in Hollywood, including her compatriot María Montez.
She was a member and supporter of the Hispanic Organization of Latin Actors (HOLA) during the organization's early days. She moved from New York and lived in Las Vegas with her fifth and final husband Rafael Flavio Echemendia. She passed away at the age of 92 on October 2, 2013, due to complications from a stroke.