His father was of Spanish ancestry, born in San Luis Potosi, Mexico and his mother was from England. As such, his talent was strongly influenced by the two cultures. At only 15 years of age, Gandara was admitted as a student of Gerome and Cabanes at the Ecole des Beaux-Arts. Soon, he was to be recognized when the jury of the 1883 Salon des Champs-Elysées singled out the first work he ever exhibited: a portrait of Saint Sebastian.
Less than ten years later, young Gandara had become one of the favourite artists of the Paris elite. His models included Countess Greffulhe, Grand Duchess of Mecklenburg, Princess of Chimay, Prince de Polignac, Prince de Sagan, Leconte de Lisle, Paul Verlaine, Leonor Uriburu de Anchorena, Sarah Bernhardt, Romaine Brooks, Jean Moreas, and Winnaretta Singer.
Influenced by Jean-Baptiste-Siméon Chardin, his own unique skill is demonstrated in his portraits, in a simplicity with the finest details, or the serenity of his scenes of the bridges, parks, and streets of Paris.
Gandara illustrated a small number of publications, including Les Danaïdes by Camille Mauclair. With James McNeill Whistler, Jean-Louis Forain, and Yamamoto, La Gandara illustrated Les Chauves-Souris of the French poet Robert de Montesquiou. The book, published in 1893, has become a rare collector's item.
The first exhibition of Gandara's work organised in New York by Durand-Ruel in 1898 was a major success and confirmed the painter as one of the masters of his time. Major newspapers and magazines routinely reproduced his portraits, several of which made the front page of publications like the fashionable Le Figaro magazine.
Although his fame faded rapidly after his death, the current growing interest in the 19th century, has seen Gandara regain popularity as a key witness to the art of his time through his canvases first, but also as the model chosen by the novelists Jean Lorrain and Marcel Proust, and through the anecdotes of his own life narrated by Goncourt, Georges-Michel, and Montesquiou.