Now, in a matter of months, he has emerged as the most notorious insurgent commander in Pakistan, blamed by authorities not only for masterminding Benazir Bhutto's assassination, but for waging a virtual one-man jihad against the government of President Pervez Musharraf.
Mahsud, who late in 2007 became the leader of Pakistan's Taliban movement, is accused of sending dozens of suicide bombers into Pakistani cities over the last year. He is also said to have unleashed a guerrilla campaign that has rattled Pakistan's powerful military and brought pitched battles to the doorstep of Peshawar, capital of the volatile North-West Frontier Province and gateway to Afghanistan and Pakistan's tribal belt.
Some observers regard Mahsud as the most potent threat to emerge in years from the tribal milieu, a leader who has shown himself capable of unifying an array of disparate homegrown groups, even while exchanging crucial logistical aid, know-how and resources with Al Qaeda. If his coalition holds firm, these observers say, he could be in a position to threaten not only Musharraf but the Pakistani state.
But the ascension of Mahsud has also prompted debate among analysts and military officials as to his true stature within the larger Al Qaeda-driven insurgency based in the tribal areas -- a network whose strength and reach has been the subject of sharp new warnings this month by senior U.S. intelligence officials.
Some observers see Mahsud as more of a figurehead, a handy scapegoat for a plethora of hostile acts unlikely to have been engineered by the same person. There are persistent suggestions, they say, that Mahsud is being used by elements within the Pakistani security establishment to further their own goals; before her death, opposition leader Bhutto herself described him as a "pawn."
Despite his growing infamy, Mahsud has preserved an aura of secrecy, speaking rarely to outsiders and scrupulously avoiding being photographed. Even his age is a mystery; he is believed to be in his 40s.
He sleeps in safe houses, makes sparing use of electronic communication, travels with an entourage of heavily armed bodyguards, and unhesitatingly orders the deaths of subordinates who fail to carry out his wishes, according to several people with access to detailed intelligence.
Those who have met him describe a surprisingly unprepossessing figure. He stands only about 5 feet tall and wears his hair long and shaggy in the manner of his Mahsud tribe. Uneducated beyond the madrasa level, he is troubled by health problems stemming from diabetes.
"He wasn't the imposing tribal type I expected," said Iqbal Khattak, a Peshawar-based journalist who has met Mahsud several times. "But in spite of his appearance, you could see that he has an authority about him."
Both Mahsud and Al Qaeda vowed to take revenge against Musharraf after government forces in July stormed a radical mosque in the capital, killing its chief cleric and dozens of his young disciples, many of whom were from South Waziristan. Even before the government assault on the Red Mosque, both groups had disdained the Pakistani leader as a tool of the West for allying himself with Washington in the fight against Islamic militants.
Estimates of the size of Mahsud's corps of fighters vary widely. His inner circle of several hundred armed followers is mainly drawn from his tribe, whose reputation as a feared fighting force dates back to British colonial times.
In mid-December, however, Mahsud assumed leadership of an umbrella group known as the Tehrik-i-Taliban Pakistan, or the Taliban movement of Pakistan, encompassing at least half a dozen militant groups and more than 20,000 fighters, officials and analysts believe.
"He's a very clever and committed fighter, a good tactician, with no qualms at all about extreme forms of violence" such as beheadings, said Mehmood Shah, a retired brigadier general with long experience in the tribal areas.
Concerns about the threat posed by both Al Qaeda and Mahsud's new coalition have prompted some of the Bush administration's most senior intelligence and military officials to travel to Pakistan to lobby for more aggressive U.S. military action.
"This is a threat to the identity and stability of the Pakistani state," CIA Director Michael V. Hayden said at a congressional hearing last week.
Analysts say Mahsud's new alliances give him a geographical reach extending far beyond the loosely governed tribal belt, now ranging from an urban base in the southern port city of Karachi to "settled" areas of Pakistan's northwest, where conflict has spilled from the tribal areas in recent months.
During an unrelenting series of attacks in recent weeks, Mahsud fielded relatively small, lightly equipped contingents of fighters, relying on surprise to overwhelm stronger conventional forces.
Rather than seeking to seize and hold territory, he and his followers made symbolic strikes meant to humiliate and demoralize government troops. In one trademark assault in mid-January, Mahsud and a force that may have numbered in the dozens overran a government-held fort in South Waziristan, blew it up, killed or captured the defenders -- and disappeared into the night.