Lachlan McIntosh eventually decided to enter business life and he moved to Charleston, South Carolina. There he was fortunate to find a position in the counting-house of Henry Laurens, who eventually became a very good friend. Laurens wielded an unusual influence upon the maturing life of Lachlan McIntosh. Through Laurens, he had a real knowledge of British policies and came to realize the policies were definitely restrictive and coercive. His military heritage led him to study military science, so when the Revolutionary War broke out, McIntosh was ready to do his part for the colonial cause.

Sometime before the revolution began, McIntosh returned to Georgia, where he secured land and became a surveyor. McIntosh was selected as a delegate from the parish of St. Andrew for the Provincial Congress in 1775, held in Savannah. On January 7, 1776, he was appointed Colonel of Georgia troops and in September 1776, he was elected Brigadier General of the Continental troops of Georgia.

Another leader in Georgia in the meantime, Button Gwinnett, was sent to the Continental Congress at the time of the adoption of the Declaration of Independence. Button Gwinnett was one of the signers of the document. In the fall of 1776, Georgia drew up a constitution and it was adopted early in 1777. Gwinnett was later made Chairman of the Council, giving him unusual power and executive prestige. As a Brigadier General of the Continental Army, McIntosh had military powers which Gwinnett could not overrule. Unfortunately, Gwinnett tried to restrict and delay plans of McIntosh, irritating him whenever possible. McIntosh finally lost all patience, denounced him and challenged him to a duel. In the duel that followed, each man was wounded. McIntosh recovered, but Gwinnett died. Gwinnett's friends charged McIntosh with murder and he was brought to trial. In the trial that followed, McIntosh was acquitted. However, the unfortunate situation divided the ranks of patriots in Georgia...and under these circumstances, influential friends secured the transfer of General McIntosh to General Washington's headquarters, accepted by McIntosh to keep the ranks of patriots united in Georgia.

At Valley Forge, Washington appointed General McIntosh the command of the North Carolina Brigade. According to General Weedon's Valley Forge Orderly Book, beginning on December 27, McIntosh is appointed a Brigadier for the day and is so appointed a successive fourteen times during the encampment. On January 30, 1778, General Washington gave a report on the result of a General Court martial held on January 23, when a soldier of the artillery was tried for desertion and stealing a horse from General McIntosh. The soldier was acquitted of the desertion charges, but found guilty of theft and was sentenced to receive 100 lashes on his back and have "half his pay stopped monthly until General McIntosh is fully satisfied".

May 26, 1778, Washington wrote a letter to McIntosh which ended his service at Valley Forge: "The Congress having been pleased to direct me to appoint an officer to command at Fort Pitt, and on the western frontiers in the room, of Brigadier-General Hand, I am indeed, but not without reluctance, from the sense I entertain of your merit, to nominate you, as an officer well qualified from a variety of considerations to answer the object they have in view, I do not know particularly what the objects are, which Congress have in contemplation in the command; and I therefore request that you will, as soon as you conveniently repair to Yorktown and receive their instructions respecting them. I have only to add, that I shall be happy to hear from you as often as opportunity will permit and my warmest wishes, that your service may be favourable to yourself and approved by your country, I am, Sir, with great esteem and regards, your most obedient servant."

After conferring with Congress, McIntosh proceeded to Fort Pitt. Congress sent three commissioners from York (Pennsylvania) to confer with McIntosh concerning the advisability of organizing an expedition against Detroit to destroy the British power in that area as well as break Indian raids between Detroit and Fort Pitt. McIntosh lacked the men to carry on such an expedition, but felt confident that he could raise forces from the frontier area. He needed money and equipment and Congress decided they just could not afford the expense.

A fort was built at the mouth of Beaver Creek at Beavertown. It was named Fort McIntosh by its French engineer, Chevalier DeCambray, who was in charge of the artillery at Fort Pitt. McIntosh marched into the Sandusky area and built another fort, Fort Laurens, named in honor of the President of the Continental Congress. He established a garrison of about one-hundred and fifty men under the command of Colonel John Gibson. McIntosh returned to Fort Pitt and remained in command until May 18, 1779 when he was ordered south to participate in a campaign to recapture Savannah.

Late in 1778, the British sent an expedition from New York to the south in order to capture Savannah and destroy the American cause in Georgia. Two days before Christmas, the British appeared before Savannah. The American forces were composed of Continental soldiers and untrained militia under General Robert Howe. When the British attacked, the militia fled and the Continentals were forced to retreat. Savannah was captured.

The citizens of Georgia resolved to recapture Savannah. Congress promised such assistance as it was possible to give. In the early fall of 1779, General McIntosh was to have an important part. The French also offered assistance. In September, Count D'Estaing appeared off the coast with a fleet of twenty ships of the line and eleven frigates...and six thousand French soldiers. D'Estaing had defeated the British in the West Indies, freeing them to aid the Americans. General Lincoln was in command of the American forces, and the he and D'Estaing arranged for immediate attack on Savannah. General McIntosh and Count Casimir Pulaski were sent in with their forces in advance of the main army under Lincoln. On September 16, D'Estaing demanded an unconditional surrender of the British forces. The British commander demanded a truce until the following day and D'Estaing agreed. Lincoln protested when he heard of his actions, but by then, it was too late. During the truce, the British fortified and prepared for attack. Also, the British were aided by the arrival of reinforcements who eluded detection under cover of heavy fog. The British sent out a note to D'Estaing in which was stated they would not surrender.

The French and Americans prepared for attack. On October 4, a terrific bombardment began, and they were well on the way to a victory. D'Estaing became impatient when he heard it would take at least ten days more to capture the city without too much loss of life. He also feared the possibility of the British fleet attack on sea, placing them in a serious situation. He resolved to destroy the British by attack and renewed the bombardment. Lincoln could do nothing but agree. After an unusually heavy bombardment and under cover of fog, the French forces led by D'Estaing in person, aided by Lincoln went in for the attack. Additional forces were led by McIntosh, Hughes and Pulaski. In the foray, both D'Estaing and Pulaski were wounded. Pulaski was taken aboard the United States ship, Wasp. He died and was buried on St. Helena Island, about fifty miles from Savannah.

As a result of the fighting, D'Estaing asked for a truce to bury the dead. The British gave four hours. D'Estaing and Lincoln held a conference where D'Estaing resolved to abandon the seige and sail away. Lincoln protested, but D'Estaing carried out his plans. Lincoln ordered his troops to his men to march to Charleston, South Carolina and McIntosh was ordered to follow.

Realizing that the capture of Savannah would lead to British attack upon Charleston, the Americans headed there. Lincoln's army had been reduced to a small force, as the term of enlistments had expired and the militia felt the service was in vain. McIntosh, however, endeavored to keep the spark of liberty alive. He was able to build up a sizable force through the inspiration of his own example. Lincoln and McIntosh were determined to protect the city. On April 9, 1780, the British fleet moved forward, pass the fire from Fort Moultrie and anchored cannon within range of Charleston. Supplies were low and the financial situation in Charleston was very poor. A forty day struggle ensued, and finally, due to the shortage of provisions and no additional troops aiding them, Lincoln agreed to a surrender. The British took possession of the city on May 12, 1780. McIntosh was taken prisoner and was not exchanged until February 9, 1782. His confinement undermined his health and he was not in robust condition for the rest of his life.

When the British withdrew from Savannah in 1782, Governor Martin of Georgia called a meeting of the state legislature which met in the house of General McIntosh, August 1782, for the reorganization of the civil authority of the state. Eventually, McIntosh returned to his Georgia estate. In 1783 he was made a Major-general. He became a member of the Society of the Cincinnati in 1784. He was elected to Congress in 1784, also, by his constituents at home. He was also appointed to help organize a treaty with the Indians that occupied the western part of Georgia.

McIntosh was a member of the committee that welcomed George Washington to Savannah in 1791. He continued to aid the state of Georgia in many ways throughout the rest of his lifetime, until his death February 20, 1806. He is buried in the Colonial Cemetery in Savannah.