In 1755, Arnold tried to join the provincial militia to fight in the French and Indian War, but his mother did not let him. In 1757, when Arnold was sixteen, he did enlist in the militia, which marched to Albany and Lake George. When Arnold's commanders heard that the French had captured of Fort William Henry, they turned around. Arnold was in the military for 13 days Some say that Arnold deserted in 1758 but there is no proof.
Arnold's mother died in 1759. Arnold's father's alcoholism got worse, so Arnold had to support his father and younger sister. His father was arrested for public drunkenness, was refused communion by his church, and eventually died in 1761.
The Lathrops helped Arnold become a pharmacist and bookseller in New Haven, Connecticut in 1762.
Arnold was hardworking and successful, and his business begame larger. In 1763, he repaid the Lathrops the money he borrowed, bought back the family house that his father had sold, and re-sold it a year later for a large profit.
In 1764 he and Adam Babcock, another young New Haven merchant, became partners. Using the profits from the sale of the family house, they bought three ships and started trading with the West Indies. Arnold brought his sister Hannah to New Haven to work in the apothecary business when he was not there. He traveled a lot for his business, throughout New England and from Quebec to the West Indies, often running of one of his own ships. On one of his voyages, Arnold fought a duel in Honduras with a British sea captain who had called him a "damned Yankee, destitute of good manners or those of a gentleman." The captain was wounded after in the first part duel, and apologized after Arnold threatened to aim to kill on the second part.
The Sugar Act of 1764 and the Stamp Act of 1765 limited trade in the colonies. Because of the Stamp Act, Arnold joined groups of people who did not like those taxes. He also joined the Sons of Liberty, a secret organization. The Sons of Liberty sometimes used violence. At first, Arnold did not go to protests in public. Like many merchants, Arnold opposed the Stamp Act by continuing his trading business without paying the tax. This meant he was a smuggler. Arnold also lost much of his money. He owed £16,000 and some creditors told people that he was bankrupt. He sued them.
On the night of January 28, 1767, Arnold and members of his crew, watched by a crowd of Sons of Liberty, attacked and beat up a man whom they thought had told the government that Arnold was smuggling. Arnold was convicted of disorderly conduct and fined the relatively small amount of 50 shillings. This sentence was very light. This might have been because many people knew about the case and agreed with Arnold.
On February 22, 1767, Arnold married Margaret Mansfield. She was the daughter of Samuel Mansfield, the sheriff of New Haven. Historians think Arnold may have met Samuel Mansfield at the local Masonic Lodge. Arnold became business partners with Samuel Mansfield. Samuel Mansfield used his job as sheriff to help Arnold stay away from people to whom he owed money. Arnold's first son, Benedict VI, was born in 1768. Richard Arnold was born in 1769. Henry Arnold was born in 1772. Margaret died early in the revolution, on June 19, 1775, while Arnold was still at Fort Ticonderoga. Even while Margaret Arnold was still alive, it was really Arnold's sister Hannah who ran their house.
Arnold was in the West Indies when the Boston Massacre happened on March 5, 1770. He wrote he was "very much shocked" and wondered "good God, are the Americans all asleep and tamely giving up their liberties, or are they all turned philosophers, that they don't take immediate vengeance on such miscreants."
Arnold began the war when he was elected as a captain in Connecticut's militia in March 1775. After the start of fighting at Lexington and Concord the following month, his company marched northeast to help at the siege of Boston that followed. Arnold told the Massachusetts Committee of Safety of his idea to seize Fort Ticonderoga in New York, which he knew was poorly defended. They made him a colonel on 3 May 1775, and he immediately rode off to the west, arriving at Castleton in the disputed New Hampshire Grants (present-day Vermont) in time to join with Ethan Allen and his men in the capture of Fort Ticonderoga. He followed up that action with a bold raid on Fort Saint-Jean on the Richelieu River north of Lake Champlain. When a Connecticut militia force arrived at Ticonderoga in June, he had a dispute with its commander over control of the fort, and resigned his Massachusetts commission. He was on his way home from Ticonderoga when he learned that his wife died earlier in June.
When the Second Continental Congress authorized an invasion of Quebec, in part on the urging of Arnold, he was passed over for command of the expedition. Arnold then went to Cambridge, Massachusetts, and suggested to George Washington a second expedition to attack Quebec City via a wilderness route through present-day Maine. This expedition, for which Arnold received a colonel's commission in the Continental Army, left Cambridge in September 1775 with 1,100 men. After a difficult passage in which 300 men turned back and another 200 died en route, Arnold arrived before Quebec City in November. Joined by Richard Montgomery's small army, he took part in the assault on Quebec City on 31 December, in which Montgomery was killed and Arnold's leg was shattered. Rev. Samuel Spring, his chaplain, carried him to the makeshift hospital at the Hotel Dieu. Arnold, who was promoted to brigadier general for his role in reaching Quebec, maintained an ineffectual siege of the city until he was replaced by Major General David Wooster in April 1776.
Arnold then traveled to Montreal, where he served as military commander of the city until forced to retreat by an advancing British army that had arrived at Quebec in May. He commanded the rear of the Continental Army during its retreat from Saint-Jean. James Wilkinson said Arnold was the last person to leave before the British arrived. He then directed the construction of a fleet to defend Lake Champlain, which was defeated in the October 1776 Battle of Valcour Island. His actions at Saint-Jean and Valcour Island played a notable role in delaying the British advance against Ticonderoga until 1777.
During these actions, Arnold made a number of friends and a larger number of enemies within the army power structure and in Congress. He had established decent relationships with George Washington, commander of the army, as well as Philip Schuyler and Horatio Gates, both of whom had command of the army's Northern Department during 1775 and 1776. However, a dispute with Moses Hazen, commander of the 2nd Canadian Regiment, boiled over into a court martial of Hazen at Ticonderoga during the summer of 1776. Only action by Gates, then his superior at Ticonderoga, prevented his own arrest on counter charges leveled by Hazen. He had also had disagreements with John Brown and James Easton, two lower-level officers with political connections that resulted in ongoing suggestions of wrongdoing on his part. Brown was particularly vicious, publishing a handbill that claimed of Arnold, "Money is this man's God, and to get enough of it he would sacrifice his country".