MEXICO CITY — As Mexican marines closed in on infamous drug lord Rafael Caro Quintero deep in the mountains of his native state of Sinaloa, it was a 6-year-old bloodhound named Max who rousted from the undergrowth the man allegedly responsible for the murder of a U.S. DEA agent more than three decades ago.

While the United States’s motivation to find Caro Quintero was never in doubt — hence the $20 million reward for information leading to his capture — there was less certainty about the commitment of the Mexican president, Andrés Manuel López Obrador, who had made clear his disinterest in pursuing drug lords.

Yet on Friday, three days after Mr. López Obrador and President Biden met at the White House, the most wanted target of the U.S. Drug Enforcement Administration was in Mexican custody.

Mexico’s Attorney General’s Office said in a statement late Friday that Caro Quintero was arrested for extradition to the U.S. and he would be held at the maximum security Altiplano prison about 50 miles west of Mexico City.

‘It seems to me that in the private talks between President Joe Biden and Andrés Manuel they surely agreed to turning over high-profile drug traffickers again, which had been suspended,” a security analyst, David Saucedo, said.

Cooperation between the DEA and Mexico’s marines had led to some of the highest-profile captures during previous administrations, but not under Mr. López Obrador, Mr. Saucedo noted.

Both presidents face domestic pressure to do more against drug traffickers. With Caro Quintero’s arrest, “Narcos are being captured again and I believe that clearly it was what was in fact needed,” Mr. Saucedo said.

The founder of the organized crime office in Mexico’s Attorney General’s Office, Samuel González, now a security analyst, said that to Mr. López Obrador’s benefit, the arrest “shows evidence that there’s no protection of capos” from his administration.

Mr. González believes Caro Quintero has long been a thorn in the bilateral relationship, but said that “without doubt” his capture was fruit of the recent negotiations in Washington.

Attorney General Garland and the U.S. ambassador to Mexico, Ken Salazar, both expressed gratitude for Mexico’s efforts to catch the man blamed for the brutal torture and murder of a DEA agent. Enrique “Kiki” Camarena, in 1985 — a case that brought a low point in U.S.-Mexico relations.

“This achievement is a testament to Mexico’s determination to bring to justice someone who terrorized and destabilized Mexico during his time in the Guadalajara Cartel; and is implicated in the kidnapping, torture, and murder of DEA agent Kiki Camarena,” Mr. Salazar said in a statement late Friday.

Mr. Garland said the U.S. government would seek his immediate extradition.

Mexico’s navy and the Attorney’s General Office led the operation deep in the mountains that straddle the border between Sinaloa and Chihuahua states, many miles from any paved road. They found Caro Quintero, with help of Max, hiding in brush in a place in Sinaloa called San Simon.

Caro Quintero came from Badiraguato, Sinaloa, the same township as Joaquin “El Chapo” Guzman, the former leader of the Sinaloa cartel, which came later. Caro Quintero was one of the founders of the Guadalajara cartel and according to the DEA was one of the primary suppliers of heroin, cocaine, and marijuana to the United States in the late 1970s.

He blamed Camarena for a raid on a huge marijuana plantation in 1984. The next year, Camarena was kidnapped in Guadalajara, allegedly on orders from Caro Quintero. His tortured body was found a month later.

Caro Quintero was serving a 40-year sentence in Mexico when an appeals court overturned his verdict in 2013. The Supreme Court upheld the sentence, but it was too late — Caro Quintero had been spirited off in a waiting vehicle.

Caro Quintero was added to the FBI’s 10 most wanted list in 2018 with a $20 million reward for his capture.

Mr. López Obrador had previously seemed ambivalent about his case.

Last year, the president said the legal appeal that led to Caro Quintero’s release was “justified” because supposedly no verdict had been handed down against the drug lord after 27 years in jail. Mr. López Obrador also depicted a later warrant for his re-arrest as an example of U.S. pressure.

“Once he was out, they had to look for him again, because the United States demanded he shouldn’t have been released, but legally the appeal was justified,” Mr. López Obrador said.

A presidential spokesman, Jesús Ramírez, said at the time, “The president was just saying that it was a legal aberration that the judge had not issued a verdict on Mr. Caro Quintero after 27 years … but he was not defending his release.”

By MARÍA VERZA and MARK STEVENSON