The charges against Joseph Manuel Hunter, 48, were made in a 37-page indictment released by US Attorney Preet Bharara on Friday.
Mr Hunter - nicknamed "Rambo" by his former US Army peers - was among six people arrested on Wednesday in Thailand.
Also named in the indictment were Timothy Vamvakias, German nationals Dennis Gogel and Michael Filter, and Polish national Slawomir Soborski.
Mr Filter and Mr Soborski have yet to be arrested.
The indictment charges that Mr Hunter recruited the four men in 2012 to be "contract killers" as part of a conspiracy to import large quantities of cocaine into the United States.
During that time, the men conspired to kill a US drug enforcement agent and a confidential informant in Africa, it says.
Deputy Thai National Police Chief Somyot Poompanmoung described Mr Hunter as a "leading drug lord".
He said the American was wanted for drug smuggling, trafficking and other international crimes.
The other suspects - two British citizens, a Slovak, a Filipino and a Taiwanese national - were arrested separately. They were not named in the US indictment and were held on separate charges.
All were expected to be extradited to New York, Pol Lt-Gen Somyot said.
"This group was considered to be a big network that spanned many countries" in Southeast Asia, he said.
A federal court in New York issued a warrant for Mr Hunter's arrest on July 17, said Priewphan Damapong, an adviser to the chief of the Narcotics Suppression Bureau. Authorities cancelled his Thai visa on Sept 19.
Mr Hunter was arrested at the Phuket Country Club after agents with the US Drug Enforcement Agency (DEA) pointed him out to Thai officers.
The former US Army sergeant trained soldiers to be snipers until he left the military in 2004, authorities say.
Earlier this year, the indictment alleges, Mr Hunter and two other former soldiers agreed to murder a DEA agent and one of the agency's confidential informers, both in Liberia, for a total of $800,000.
The plot had been proposed by men posing as Colombian drug traffickers, the New York Times reported.
"My guys will handle it," Mr Hunter wrote in an e-mail on May 30.
In fact, the authorities said, the purported drug traffickers were confidential sources for the DEA and part of a sting operation.
"The charges tell a tale of an international band of mercenary marksmen who enlisted their elite military training to serve as hired guns for evil ends," said prosecutor Preet Bharara.
Mr Hunter referred to contract assassinations euphemistically as "bonus work" or "bonus jobs", the indictment says, adding that he told the confidential informers that he had done such work before.
Bharara said Mr Hunter had arranged successfully for the murders "of numerous people", though he did not name them.
The indictment says Mr Hunter began collecting resumes for prospective members of his "security team", which had planned to use pistols and submachine guns, with silencers, to carry out the murders.
Former German army corporal Dennis Gogel, 27, also named in the indictment, told one of the DEA informers that the murders could be made to resemble an ordinary street crime, "like a bad robbery or anything, you know".
Mr Hunter told his co-conspirators that they would be working for a Colombian cartel and that they could expect to "see tons of cocaine and millions of dollars", the indictment says.
They would also have the opportunity to participate in assassinations, he told them, according to the indictment. "Most of the bonus work is up close ... because in the cities ... you don't get long-range shots."
Part of an escape plan involved the use of latex face masks that would make the wearer appear to be of another race, the indictment said.
Mr Vamvakias and Mr Gogel were each sent to the United States from Liberia and arraigned on Thursday, when they were ordered detained and entered not-guilty pleas, the authorities said.
The case, with its use of confidential informers posing as drug traffickers, had echoes of other DEA international sting operations, including the one that ensnared the Russian arms trafficker Viktor Bout, who was brought from Thailand to the United States and tried and convicted in 2011.