He was born in Paris, France, but his father was of Spanish ancestry, born in San Luis Potosi, Mexico, and his mother was from England. La Gandara's talent was strongly influenced by both cultures. At only 15 years of age, Gandara was admitted as a student of Jean-Léon Gérôme and Cabanes at the Ecole des Beaux-Arts. Soon, he was recognized by the jury of the 1883 Salon des Champs-Elysées, who 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, the Grand Duchess of Mecklenburg, the Princess of Chimay, the Prince de Polignac, the Prince de Sagan, Charles Leconte de Lisle, Paul Verlaine, Leonor Uriburu de Anchorena, Sarah Bernhardt, Romaine Brooks, Jean Moreas, and Winnaretta Singer. Influenced by Chardin, his unique skill is demonstrated in his portraits, in a simplicity with the finest detail, or in 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 ("The Bats") by 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. Gandara participated in the most important exhibitions in Paris, Brussels, Berlin, Dresden, Barcelona and Saragossa.

He died on June 30, 1917, and was interred in Père Lachaise, Paris, France. Although his fame faded rapidly after his death, growing interest in the 19th century saw Gandara regain popularity as a key witness to the art of his time, not only through his canvases, but also as the model chosen by the novelists Jean Lorrain and Marcel Proust, and through the anecdotes of his own life narrated by Edmond de Goncourt, Georges-Michel, and Montesquiou.