Mercader was born in 1913 in Barcelona, but grew up in France with his mother, Eustaquia María Caridad del Río Hernández, after she separated from his father, Pau Mercader Marina. Cuban-born Caridad was an ardent Communist who fought in the Spanish Civil War and served the Soviet international underground. As a young man, Mercader embraced Communism, working for leftist organizations in Spain during the mid-1930s. He was briefly imprisoned for his activities, but was released in 1936 when the left-wing coalition Popular Front won in the elections of that year. During the civil war in Spain, Mercader was recruited by NKVD officer Nahum Eitingon and trained in Moscow as a Soviet agent.

Mercader's contacts with and befriending of Trotskyists began during the Spanish Civil War. George Orwell’s biographer Gordon Bowker relates how English communist David Crook, an ostensible volunteer for the Republican side, was sent to Albacete where he was taught Spanish and also given a crash course in surveillance techniques by Ramon Mercader. Crook then, on orders from the NKVD, used his job as war reporter for the News Chronicle to spy on Orwell and his Independent Labour Party comrades in the POUM Trotskyist militia.

Murder of Trotsky

In 1938, while he was a student at the Sorbonne, in France, Mercader befriended Sylvia Ageloff, a confidant of Trotsky in Paris, assuming the identity Jacques Mornard, supposedly the son of a Belgian diplomat. A year later, he was contacted by a representative of the "Bureau of the Fourth International." Ageloff returned to her native Brooklyn, in New York, in September that same year, and Mercader joined her, assuming the identity of Canadian Frank Jacson. He was given a passport which had originally belonged to a Canadian citizen named Tony Babich, a member of the Spanish Republican Army during the Spanish Civil War. Babich's photograph was removed and Mercader's was inserted in its place.

Mercader explained to Ageloff that he purchased forged documents to avoid military service. In October, Mercader, ostensibly to tend to business affairs (a cover provided by Eitingon), moved to Mexico City, where Trotsky lived with his family, and persuaded Ageloff to join him. Through her, Mercader began to meet with Trotsky personally, posing as a supporter of Trotsky's ideas.

The then leader of the main American Trotskyist party, Joseph Hansen, was later accused by Trotsky's followers as being "a double agent of the FBI and the GPU" and of having assisted Mercader to penetrate Trotsky's inner circle of friends and acquaintances, an accusation denied by Hansen.

On 20 August 1940, Mercader attacked and fatally wounded Trotsky with an ice axe, while the exiled Russian was in the study at his home in Coyoacán (then a village on the southern fringes of Mexico City). The blow failed to kill Trotsky instantly. Hearing the commotion, Trotsky's guards burst in and nearly killed Mercader, but Trotsky, heavily wounded but still conscious, ordered them to spare his attacker's life, shouting "Do not kill him! This man has a story to tell."

Caridad and Eitingon, having arrived in Mexico City shortly after Mercader, were waiting outside the compound in separate cars to provide a getaway; but, when Mercader didn't return, they left and fled the country.

Trotsky was taken to a hospital in the city and operated on but died the next day, as a result of "severe brain damages".

Mercader was turned over by Trotsky's guards to the Mexican authorities, to whom he refused to give his real identity. He would only identify himself as "Jacques Mornard". Mercader claimed to the police he had wanted to marry Ageloff, but Trotsky had forbidden the marriage. He alleged that a violent quarrel with Trotsky had led to him wanting to murder Trotsky. He stated that "... instead of finding myself face to face with a political chief who was directing the struggle for the liberation of the working class, I found myself before a man who desired nothing more than to satisfy his needs and desires of vengeance and of hate and who did not utilize the workers' struggle for anything more than a means of hiding his own paltriness and despicable calculations.... It was Trotsky who destroyed my nature, my future and all my affections. He converted me into a man without a name, without country, into an instrument of Trotsky. I was in a blind alley.... Trotsky crushed me in his hands as if I had been paper."

It was not until September 1950 that fingerprint evidence conclusively proved the assassin's true identity. Nevertheless, Mercader, in 1940, was convicted of murder and sentenced to 20 years in prison.

Ageloff was initially arrested by the Mexican police as an accomplice, as she had lived together with Mercader on and off for about two years, up to the time of the assassination, but charges were quickly dropped.

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