Thomas Gage was born in Firle, Sussex, England in 1719. He was the second son of the 1st Viscount Thomas Gage. In 1728, he and his brother went to the Westminster School. There, he knew John Burgoyne, George and Richard Howe, future Governor of Massachusetts Francis Bernard and George Sackville, who would go on to become Lord George Germain, the British Secretary of State of the American Colonies during the American Revolutionary War.

After leaving Westminster, Gage was commissioned an ensign in the British Army. In January 1743, he was promoted to Captain. In 1745, he saw action at Fontenoy and Culloden while serving as Aide de Camp to Albemarle. Gage participated in 1747-48 campaigns in the Low Countries. He then transferred to the 55th Regiment under Colonel John Lee. The regiment was soon renumbered the 44th. In 1748, he purchased the rank of Major and on March 2, 1751, he was promoted to Lieutenant Colonel. During this time, Gage began a long friendship with Charles Lee, a fellow officer and son of the regimental commander. James Wolfe, who would go on to great fame in fighting in Canada, was also a friend. A common-law marriage came to an abrupt end with his wife's death and a failed political foray was attempted shortly before he was ordered to America.

French and Indian War: 1754-1758

In Fall 1754, the 44th Regiment was sent to America to be part of Maj. General Edward Braddock's expeditionary force. Also serving in the British Army as officers on that expedition with Gage had been future foes George Washington, Horatio Gates and Charles Lee. Gage commanded the advance guard on July 9, 1755 and slightly wounded, when the force was obliterated by a French and Indian force in what has been alternately called the Battle of the Wilderness, Battle of the Monogahela or Braddock's Defeat. He had taken command of the 44th following the death of Colonel Peter Halkett during the fighting. Gage was accused of poor field tactics by Braddock's Aide de Camp, Captain Orme. Nothing came of the accusations although Gage was denied gaining the colonelcy of the 44th as a permanent appointment.

Gage spent winter quarters at Albany, New York, although he visited New York City during this time. In August 1756 he was second-in-command on an unsuccessful expedition up along the Mohawk River and then again wintered in Albany. In 1757 he went to Halifax, Nova Scotia with British Commander-in-Chief in America Captain-General John Campbell Loudoun. In December 1757, Gage was given authority to organize provincial troops into what was designated the 80th Regiment and qualified Gage for promotion to Colonel at last. While recruiting provincials, he courted the woman that he would eventually marry.

On July 7, 1758, he was again slightly wounded while leading a portion of Maj. General James Abercromby's advance guard in the failed attempted to capture Fort Ticonderoga. In the winter of 1758, Gage again went to New York City and met with old acquaintance Maj. General Jeffery Amherst, who in September had replaced Loudoun as British Commander-in-Chief in America. Gage also learned that his brother had secured him a temporary promotion to Brigadier General.. On December 8, 1758, Gage married Margaret Kemble of Brunswick, New Jersey. She was the daughter of Peter Kemble, whom Gage had known from his days at the Westminster School.

French and Indian War: 1759-1763

In January 1759, Brig. General Thomas Gage and his new wife arrived in and took command of Albany and its neighboring posts. He served on Maj. General Jeffery Amherst's staff until July 1759 when he was given command of forces operating on Lake Ontario with orders from Amherst to secure Fort La Galette and even putsh down and capture Montreal. Gage felt that he lacked the strength to do so and thought that he should strengthen Niagara and Oswego while Amherst and Maj. General James Wolfe moved into Canada. Amherst showed his displeasure in Gage's assessment. Gage again commanded Albany during the winter of 1759 and then commanded General Amherst's rear guard in his advance to Montreal in 1760.

Following the French surrender in Canada, Gage became military governor of Montreal and a district to the south on September 6, 1760. He did not like Catholics or Indians, but proved do do well with his post during the three years until the end of the French and Indian War. He was promoted to Major General in 1761. In June 1762, he was given command of the 22nd Regiment. On November 16, 1763, he arrived at New York City and assumed duties as acting Commander-in-Chief in America with General Amherst returning to England.

In Between the Wars: 1763-1774

On November 16, 1764, Gage officially succeeded Amherst. He would oversee the rise in political tensions in the American colonies. He had to cope with the fallout of the actions of Parliament back in London. Following the passage of the Stamp Act, Gage began to draw troops from western and rural posts into the coastal cities, especially New York City. Following the reaction to the Townshend Acts, Boston became the seat of the rebellion. Gage responded by sending troops there.

Gage soon established a garrison at Boston and more and more troops were stationed there as tensions rose. It was not long before there were too many troops, because additional troops had been ordered to Boston from Halifax, Nova Scotia by the British government in England. The Quartering Act was passed in 1765 to provide housing for the troops. On October 15, 1765, Gage went to Boston for about six weeks to make quartering arrangements for the troops. Tensioned boiled over with the Boston Massacre on March 5, 1770. However, the situation was diffused and the rebellion calmed down for a period.

During this period of calm in October 1772, Gage requested a leave of absence to return to England for the first time in seventeen years. On June 8, 1773, he and his family sailed for England. When he was in England, the Boston Port Bill was passed, closing Boston's harbor until reparations were made for the tea destroyed in the Boston Tea Party. Royal Governor Thomas Hutchinson was about to go to England on leave and to report on the situation. Royal Lt. Governor Andrew Oliver was frail and expected to die soon. As a result, it was thought that Gage's return to the American colonies was necessary.

The Coming of War: 1774-1781

On April 2, 1774, Gage was made Royal Governor of Massachusetts. On May 13, 1774, he returned to Boston and disembarked on May 17. He almost immediately declared martial law in response to the simmering tensions following the Intolerable Acts. Late in 1774, Gage began seizing powder stores, because he knew that emotions were rapidly building toward violence. On April 14, 1775, Gage received orders from London to take decisive action to halt the building rebellion. He could not arrest members of the Massachusetts Provincial Congress, because it adjourned on April 15. He instead decided to seize powder and supplies at Concord, whose location he knew about through his informants.

On April 19, 1775, General Gage ordered Lt. Colonel Frances Smith and Major James Pitcairn to carry out the seizure of the munitions at Concord, Massachusetts. He wanted it done in secret, so as to not alert the rebels and allow them time to hide the munitions. The British troops were roused in the middle of the night and not told of their mission until they were ready to march. In spite of these precautions, word quickly spread, because Boston was full of informants and rebel spies. While the British were marching out of the city, Paul Revere and William Dawes were riding ahead to warn the rebels. Though Revere was captured and Dawes was turned back, Samuel Prescott got through to Concord and the munitions were safely hidden away. The resulting skirmishes at Lexington and Concord started the American Revolutionary War.

As the British troops returned from Concord, farmers were moving in to surround Boston. On April 23, 1775, the Massachusetts Provincial Congress voted to call up militia to form an army. On May 25, Major Generals William Howe, Henry Clinton and John Burgoyne arrived from England and any one of them was a likely candidate to replace Gage as Commander-in-Chief of the British Army in America. By June 15,000 men surrounded Boston, while Gage was in command of 6,500 troops. On June 12, 1775, Gage had offered to pardon all but Samuel Adams and John Hancock. He then decided to occupy Dorchester Heights which oversaw Boston Harbor, but on June 13, the Americans learned of his plan.

By the night of June 16, 1775, the Americans were building ramparts on Breed's Hill and when General Gage awoke on the morning of June 17, he was surprised to find significant American fortifications had been built overnight. He immediately ordered Maj. General William Howe to sail to Charleston peninsula and dislodge the rebels. In the Battle of Bunker Hill, Howe was able to dislodge the Americans only after three charges up the hill, absorbing tremendous losses, and the waning supply of ammunition for the Americans.

On September 26, 1775, General Gage received orders to return to England under the pretext of helping plan operations for the following year. On October 10, he surrendered command of the British Army in America to General Howe. On November 14, 1775, he reached London. On April 18, 1776, he was finally informed of his removal from command by British Secretary of State for America, Gage's old classmate from the Westminster School, George Sackville Germain. While he retained his rank, he was in virtual retirement while Germain remained in power for the remainder of the war.

After the War: 1781-1787

In April 1781, Maj. General Thomas Gage returned to duty with an appointment to General Jeffrey Amherst's staff with the mission to organize militia in preparation for possible attack from France. In April, 1782, he was appointed colonel of the 17th light dragoons. On November 20, 1782, he was promoted to full General. In 1785, he was transferred to the 11th dragoons. His health had begun to decline and after a long illness, he died in Portland, England on April 2, 1787. His wife would survive him by nearly thirty-seven years.