The dangers of frontier life were brought home to Lachlan in 1737 when his younger brother Lewis McIntosh was killed by an alligator while swimming in the river.

It was a highly militarized colony, as clashes with neighboring Spanish Florida and its fortress city of St. Augustine to the south were common.

His elder brother, Colonel William McIntosh, served under Oglethorpe and helped to repulse a Spanish invasion of the colony.

Lachlan and his brother William planned to travel to Scotland and join the rebellion, but General James Oglethorpe, who had become a friend and mentor to the young McIntosh, convinced them to remain in Georgia.

[5] In 1748, McIntosh moved to Charleston, South Carolina, where he took a position as a clerk for Henry Laurens, a wealthy merchant and slave trader.

On October 22, 1776, McIntosh ordered his brother William to construct a fort on the Satilla River to protect Georgia from Florida.

During the period of 1776 to 1777, McIntosh became embroiled in a bitter political dispute with Button Gwinnett, the Speaker of the Georgia Provisional Congress and a radical Whig leader.

Their bitter personal rivalry began when McIntosh succeeded Gwinnett as commander of Georgia's Continental Battalion in early 1776.

Gwinnett, thwarted in his military ambitions, became a delegate to the Continental Congress and a signer of the United States Declaration of Independence.

In addition, Gwinnett had ordered Lachlan McIntosh to lead a poorly planned military expedition into British Florida.

On May 16, in a field owned by James Wright a few miles east of Savannah, Gwinnett and McIntosh met to duel with pistols.

George Washington, fearing Gwinnett's allies would take revenge on McIntosh, ordered him to report to Continental Army headquarters on October 10.

He spent the winter of 1777–1778 with the Continental Army at Valley Forge, Pennsylvania, where he commanded several regiments of North Carolina troops.

The war was brought to an end in 1783 with the Peace of Paris which recognized American independence and transferred East and West Florida to Spain.

McIntosh was an original member of the Society of the Cincinnati[8] and in 1791 he was part of the delegation that officially welcomed President George Washington to Georgia.