The jurors at the trial of Joaquín Guzmán Loera, known as El Chapo, were treated last week to a cinematic narrative about the early years of the kingpin’s career, detailing his rise from a young upstart in the drug trade to a wealthy and successful narco-entrepreneur.

Much of the tale was told by one of El Chapo’s first employees, Miguel Angel Martínez, who began working for the cartel as a pilot in 1987 before being promoted to running operations in Mexico City.

Over four days last week as a government witness in Federal District Court in Brooklyn, Mr. Martínez described how the crime lord went from being a novice trafficker with a staff of only 25 people to earning hundreds of millions of dollars that he spent on extravagances like a fleet of private jets and a rural ranch with a zoo where guests could ride a train past crocodiles and bears.

But as with many drug-world relationships, the bond between the two men ultimately soured. In this case, that occurred in 1998, after Mr. Martínez was arrested.