Mercader was born in 1914 in Barcelona, but spent much of his youth with his mother in France after his parents broke up. As a young man, he embraced Communism, helping leftist organizations in Spain during the mid-1930s. He was briefly imprisoned for his activities, but was released when a left-wing government took control of the country in 1936.
By this time, his mother had become a Soviet agent herself, and Ramon followed in her footsteps, traveling to Moscow shortly before the outbreak of the Spanish Civil War to train in the arts of sabotage, guerrilla warfare and assassination. He was given the codename "GNOME" by his superiors.
His bosses at the NKVD selected him to assassinate Trotsky, who had left the USSR many years before after losing a power struggle to Stalin, but who had continued to antagonize the dictator with his writings from exile. In October, 1939, Mercader slipped into Mexico with a fake passport identifying himself as "Frank Jacson", a businessman.
An elaborate trap set by Mercader and other NKVD operatives in Mexico failed on May 24, 1940, and a second attempt was planned. This time, "Jacson", who had avoided raising suspicion during the first attempt on Trotsky's life, befriended an unmarried secretary of Trotsky's. Through her, he began to meet with Trotsky personally, in the guise of being a Canadian supporter of Trotsky's ideas. On August 20, Mercader fatally wounded Trotsky with an ice pick at the home of painter Frida Kahlo. Trotsky's guards busted in and nearly killed Mercader, but their leader ordered them to spare his life, yelling "Do not kill him! This man has a story to tell."
He was turned over to the Mexican authorities, to whom he refused to give up his real identity. Nevertheless, he was convicted of murder and sentenced to 20 years in prison. It wasn't until August, 1953 that his true identity was discovered, and his NKVD connections never were until after the fall of the Soviet Union.
He was released from prison on May 6, 1960 and moved to Havana, where Fidel Castro's Communist government welcomed him. In 1961, he moved to the USSR and was awarded the Hero of the Soviet Union medal, one of the country's highest decorations. He split time between Cuba and the USSR for the rest of his life, revered by the KGB (the successor to the NKVD), and died in Havana in 1978.
He is buried in Moscow's Kuntsevo Cemetery and has a place of honor in the KGB's museum in the Russian capitol.