After being passed over for a promotion (due in part to his ineptitude as a recruiter in New York), Ames applied for and was assigned to a post in Mexico City, Mexico. His wife, however, stayed behind. Met with continued lack of success as a recruiter in Mexico City, prompting him to fall further into his drinking habit. Also was becoming disillusioned with certain activities by the CIA in Latin America.

Met Columbian socialite Maria del Rosario Casas, the cultural attache for the Columbian embassy in Mexico City. Her father was a former member of the Columbian Senate and she was raised in a world of privilege. She graduated Summa Cum Laude from the University of the Andes in 1974 and became a faculty member at the University shortly thereafter. Was extremely close to her mother, Cecelia Depuy de Casas, a lover of music and also a faculty member at the University. Rosario had on occasion loaned out her apartment to CIA operatives for meeting with Mexican spies. After being introduced to Ames, she was recruited by him into service with the CIA. The two were romantically involved and traveled among the more prominent circles in Mexico.

Ames was promoted in 1983, heading up the CIA Soviet counterintelligence branch, and was assigned to CIA headquarters in Langley, Virginia. In his new position, he was given access to nearly all information available on Soviet cases, including U.S. assets operating within the Soviet Union. After a period of time, he brought Rosario to the United States to live with him. She pressured him to divorce his wife Nancy and she consented, keeping most of the couples assets (the divorce was finalized in 1985). Rosario began running up huge bills, shopping and placing calls to her mother in Bogota. Ames was soon almost $60,000.00 in debt, but was earning only $45,000.00 each year. Burdened by his overwhelming debt-load, Ames began searching for means of obtaining additional money. He had once heard that a co-worker had been offered $50,000 to spy for the KGB, and he began to consider that as a possibility.

In April 1985, Ames tried to meet with Sergey Chuvakhin, a Soviet arms expert to suggest that he might be willing to spy for the Soviet Union. Instead, he decided to approach Stanislav Androsov, a Soviet agent at the Soviet embassy. Ames passed Androsov a note, offering to provide the name of three Soviets working for U.S. intelligence in exchange for $50,000.00. Androsov introduced him to Victor Cherkashin, the KGB counterespionage chief at the embassy. Cherkashin accepted Ames' offer and Ames was given a bag containing $50,000.00.