When the seven light infantry companies reached the North Bridge, four companies marched off to the Barrett farm. The remaining three companies deployed near the bridge to secure it for the return of the search party. Major Buttrick, whose forces now outnumbered the regulars, advanced his men to a position 400 yards from the bridge and 50 feet above. This forced the regulars to withdraw closer to the bridge, on low ground with their backs to the river, a tactically weak position. While both groups maintained a wary distance, additional men continued streaming in from surrounding communities to the north and west of Boston. By about 10:30, Colonel Barrett had perhaps 500 men under his command. The light infantry at the bridge, numbering about 100 men, eyed Barrett's men and noted that "their disposition appeared to be very regular and with Barrett and Buttrick, determined." The regulars sent an officer back to Colonel Smith to ask for additional men.

Looking from the ridge above the town, a pale column of smoke rose over the trees caught the eyes of Lieutenant Joseph Hosmer, Colonel Barrett's second in command and acting adjutant of the Concord regiment. Turning to Barrett he exclaimed, "Will you let them burn the town down?" The militia loaded their muskets.The move would require them to cross the North Bridge, which at the time was being guarded by three companies of regulars  totaling 120 men. Barrett had 400 militia behind him and thought that by advancing on the bridge, the regulars, facing such an overwhelming force, would turn and fall back to allow the Americans to proceed on into Concord. Under the current rules of engagement, the British would not fire unless the Americans fired first.

Barrett gave the order to march in its defense and Major Buttrick, alongside Capt. Isaac Davis, Acton (the lead company) to advance his  company to the bridge in a long, snaking column, two men abreast. Barrett cautioned them to be sure not to fire first. Seeing Buttrick's minutemen marching toward them, the regulars retreated to the North Bridge and started to cross back over it. As the  Americans approached the bridge, the stunned British at first did nothing. When their commanding officer, Captain Walter Laurie, realized the situation, he had his men retreat to the opposite side of the river and massed them around a narrow span. They had to hold the bridge or the four companies that had marched to Barrett's farm would be cut off.

Barrett rode along the line exhorting his men not to fire first but fire fast if attacked. The Americans advanced. The British raised their muskets. The Americans marched onto the bridge. With their guns pointed down and toward the river, several British soldiers fired warning shots. The American kept coming. Suddenly, an instant later, a full volley was fired at the head of the American column. Captain Issac Davis was killed instantly with a bullet in his heart. Beside him, Abner Hosmer went down with a bullet to the head. Four other men were wounded.

The Americans stared in disbelief. The American command to fire has historically been attributed to Buttrick who according to tradition leaped from the ground, partly turned to his men, shouted "Fire fellow  soldiers, for God's sake, fire!", then discharged his musket. He watched the British volley at the column and then his men return fire, routing the enemy. Quickly he moved to regain control of his units and after a short respite, returned the older militia men to the West side of the Bridge while the minute men took cover on an easterly hill behind a wall. The provincial volley and subsequent shooting killed two regulars and wounded nine more, including four of the eight Crown officers. The regulars broke ranks under the provincial fire and ran back toward Concord. After rallying the men encouraging them to pursue the retreating British at Merriam's Corner, he removed himself from the scene possibly to check the wounded, visit the town and see to the well-being of his family.

The British Retreat back toward Lexington but are ambushed at (Merriam'sCorner)