Geagea was born in 1952 in the Ain Roumaneh neighborhood of Beirut to a family of modest means from the northern Lebanese village of Bsharri. The son of an adjutant in the Army, Geagea came of age at a time when the barriers to socio-economic advancement within the Christian community had begun to weaken and record numbers of students were arriving at universities on the strength of their intelligence and self-discipline, rather than wealth or family connections. Geagea was one of them, arriving at American University of Beirut (AUB) to study medicine in 1972.

AUB, the birthplace of political movements ranging from the Syrian Social Nationalist Party (SSNP) to the Popular Front for the Liberation of Palestine (PFLP), was a hotbed of activism in the early 1970s. Although Geagea had been active in the student branch of the Kata'ib (Phalange) party when he was in high school, it was here that he found his leadership calling.

After the outbreak of civil war in 1975, Geagea interrupted his studies to participate in the defense of Christian towns and villages from Palestinian attack. Although he would later complete his studies at the University of St. Joseph, Geagea never practiced medicine - the massacres and dislocations experienced by the Christian community in the early war years impelled him to devote his career to their defense. As the Lebanese Army splintered and government authority crumbled, Geagea proved himself to be a fearless soldier and able leader, quickly rising through the ranks of Bashir Gemayel's Kata'ib militia and its successor, the Lebanese Forces (LF).

The Palestinian threat to Lebanon had been counteracted to a certain extent by the end of 1976, but the Christian community faced an even more powerful threat with the entry of Syrian forces into Lebanon that year. While the Kata'ib staunchly opposed Syrian intervention, some Christian leaders who had steadfastly fought (or sent their followers to fight) the PLO's attempted takeover of the country were perfectly willing to accommodate Syria's hegemonic ambitions so long as they obtained a share of the post-war political spoils. Former President Suleiman Franjieh, whose militiamen fought bravely against Palestinians with whom he had no financial interests, defected from the Christian alliance because of his long-standing business ties to Syrian President Hafez Assad. By 1978, Franjieh's Zghorta-based militia, commanded by his son, Tony, was coordinating directly with Syrian military intelligence and waging a relentless wave of terrorism, ambushes, and assassinations against the Kata'ib throughout north Lebanon. When a local Kata'ib leader, Joud al-Bayeh, was murdered by a Franjieh assassination squad on June 8, Gemayel tried to settle the problem through negotiations via Maronite Patriarch Antonios Khreich. When these negotiations failed, Gemayel decided to retaliate with a reprisal raid deep into the warlord's domain and hand-picked a special force to carry it out. One of the units was led by 26-year old Geagea, whose hometown was traditionally at odds with the Franjieh clan. The plan was to arrest Joud al-Bayeh's assassins, who were seeking protection and refuge in Franjieh's palatial summer residence in Ehden, a symbol of the family's prestige and a major arsenal and communications center. On the evening of June 12, Geagea's task force infiltrated the area at night and began attacking the compound just before dawn. The defenders refused to surrender and a long gun battle ensued in which Geagea was seriously injured and fell unconscious on the road leading to the compound. The operation involved close house to house combat and was successful from a military standpoint, but when the smoke cleared and Gemayel's men entered the compound, they unexpectedly discovered among the dead Tony Franjieh and several members of his family in one of the guards' hangars (the warlord's unwillingness to surrender in spite of the imminent danger to his family has remained an enduring mystery).

After recuperating at a hospital in France, Geagea returned to Lebanon and was appointed commander of LF forces in north Lebanon. Over the next several years, he fortified LF outposts, expanded recruitment and built new training centers. More importantly, he earned the unswerving loyalty of roughly 1,500 militiamen under his direct command. Most, like Geagea, had been dislocated from their villages and towns in areas of north Lebanon controlled by Syria and its militia allies - they lived in barracks, unlike LF soldiers in east Beirut, who could return to their homes each night. Having tasted insecurity so acutely, Geagea and his followers viewed the security of the Christian community, not its political share of the post-war spoils, as their top priority.

Lebanon's First Republic had failed to provide this security. The LF's main function was to fill the security void left by the breakdown of the army and government administration - a mandate that also necessitated the development of a highly organized civil infrastructure. Unlike their counterparts in Syrian-occupied Lebanon, inhabitants of the LF-ruled enclave enjoyed modern healthcare, affordable public transport, welfare support, and personal security. What little prosperity the Lebanese Christian community still enjoys today is largely due to the LF's success in preserving an environment in which children could still go to school - in sharp contrast to West Beirut, where the rule of Muslim militias placed guns, not books, in children's hands.

Bashir Gemayel's election as president following the Israeli invasion of Lebanon in 1982 briefly revived public hopes that the First Republic could be fixed. These hopes were shattered after Bashir's assassination and the ascension of his brother, Amin, who invited American and European peacekeepers to the capital to support his government. Geagea and other LF leaders staunchly backed President Gemayel so long as he remained committed to the withdrawal of Syrian forces, but the withdrawal of American and European peacekeeping troops in February 1984 led the president to seek rapprochement with Damascus. Moreover, Gemayel attempted to strengthen his bargaining hand in negotiations with Syria by asserting control over the LF. In November, the president succeeded in securing the replacement of LF chief Fadi Frem with his nephew, Fouad Abi Nader. However, a faction of the LF headed by Geagea and LF intelligence chief Elie Hobeiqa sidelined Abu Nader and took control over the Christian enclave in March 1985.

Hobeiqa soon made an astonishing political turnabout of his own, aligning himself with Damascus in hopes of reaching an accord with Syrian-backed militias and assuming the presidency in a Syrianized post-war republic. In spite of widespread Christian opposition, Hobeiqa signed the December 1985 Tripartite Accord, a Syrian-brokered agreement that would have legalized the Syrian presence in Lebanon. In response, LF forces loyal to Geagea swiftly took control over the Christian enclave and Hobeiqa fled to Syrian-occupied territory, nursing an intense personal hatred of Geagea.

Geagea's ability to mobilize the LF rank and file twice against those who sought to accommodate Syria's hegemonic ambitions had much to do with his incorruptibility. Unlike other "warlords" in Lebanon, Geagea had "an almost puritanical disdain for material concern," notes historian Theodor Hanf in his voluminous study of the war. Even Washington Post correspondent Jonathan C. Randal, who is scathingly critical of Maronite militia leaders in his best-selling book on the war, described Geagea as "well-read, thoughtful, and possessed of a revolutionary soul."

At the time, Geagea's defiance of Damascus appeared risky. By the mid-1980s, the LF had lost its principal external patron (Israel), the Christian community's financial strength had been devastated by the collapse of the Lebanese economy, American interest in supporting Lebanon had dropped to nil, and Syrian forces or their militia allies had gained control of most of the country. However, Geagea managed to defend the Christian enclave and maintain close relations with the United States. As Lebanon's Muslim militias turned on each other with a ferocity not seen in Lebanon since the height of the war in 1976, residents of the Christian enclave went about with their lives as best they could.