Eaton observes a fort in Derne with eight guns on the morning of April 27.

The brig USS Argus sends a cannon ashore to use in the attack.

Captain Hull's ships then open fire and bombard Derne's batteries for an hour.

Eaton meanwhile  divides his army into two separate attacking parties.

Hamet is to lead the Arab mercenaries southwest to cut the road to Tripoli, then attack the city's left flank and storm the weakly defended governor's palace.

Eaton with the rest of the mercenaries and the squad of Marines are to attack the harbor fortress.

Hull and the ships will fire on the heavily defended port batteries.

The attack begins at 2:45 p.m., with Lt. O'Bannon and his Marines leading the advance.

O'Bannon leads his Marines and fifty Greek gunners with the field piece from the Argus, though the gun's effectiveness is lessened after the firing crew carelessly leaves the ramrod in the tube and fires it down range.

The harbor defenses have been reinforced, and the attackers are temporarily halted, but this had weakened the defenses elsewhere and allow the Arab mercenaries to ride unopposed into the western section of the city.

Eaton's mercenary army is hesitant under the enemy's musket fire, and he realizes a charge is the only way to regain the initiative.

Leading the charge, he is seriously wounded in the wrist by a musket ball.

On the Argus, Captain Hull sees the Americans and mercenaries are "gaining ground very fast though a heavy fire of Musquetry [sic] was constantly kept upon them."

The ships cease fire to allow the charge to continue.

Eaton will report that O'Bannon with his Marines and Greeks "pass'd through a shower of Musketry from the Walls of houses, took possession of the Battery".

The defenders flee in haste, leaving their cannons loaded and ready to fire.

O'Bannon raises the American flag over the battery (the unique fifteen stars - fifteen stripes emblem used 1795-1818, later made famous in the War of 1812 as the "Star-Spangled Banner"), and Eaton turns the captured guns on the city.

Hamet's force has seized the governor's palace and secured the western part of the city.

Many of the defenders of the harbor fortress flee through the town and run into Hamet's force.

By 4:00 p.m. the entire city has fallen, and for the first time in history, an American flag flies over fortifications on the opposite side of the Atlantic Ocean.

According to Spencer Tucker, casualties during the fighting for the Americans are two killed and three wounded, while those among the Christian / Greek mercenaries were nine killed or wounded. (Tucker, Spencer, ed. (2014). The Encyclopedia of the Wars of the Early American Republic, 1783–1812: A Political, Social and Military History. Volume I: A–K. Santa Barbara, California: ABC-CLIO. p. 167.)
 
Muslim Turkish / Arab mercenary casualties are unknown, as are those of the defenders.

Yusuf in Tripoli to the west is aware of the attack on Derne and had sent reinforcements to the city.

By the time this force arrives, however, the city has fallen. His men dig in and prepare to recapture the city.

Eaton fortifies his new position, while Hamet takes up residence in the governor's palace and has his Arabs patrolling the outer areas of the city.

Yusuf's men dig in south of the city and wait.