The 1638-1651 Wars of the Three Kingdoms created strong resistance in Scotland and England to a permanent military; this meant those who wanted to pursue a military career often did so in foreign armies.
When the Third Anglo-Dutch War began in 1672, the Duke of Buckingham was authorised to recruit a regiment for service against the Dutch.
Thomas' cousin James was a lieutenant in this unit and it is probable Thomas served with him.

When England and the Netherlands made peace in the January 1674 Treaty of Westminster, these recruits enrolled in the Anglo-Scots Brigade, a mercenary unit in Dutch service.
The first confirmed record of Thomas' service is his commission as captain in 1688 but he is referred to in 1698 as having '24 years service;' this makes it likely he saw action during the 1672-1678 Franco-Dutch War.
His movements can be traced by the birth of his first two children in the Dutch port of Heusden between 1687 to 1689, while the others were born in Ireland between 1692 to 1694.

The Brigade accompanied William III to England in the November 1688 Glorious Revolution; Thomas served in the 1689-1691 Williamite War in Ireland, including the Battle of the Boyne.
After the 1691 Treaty of Limerick, he was among those transferred to Flanders during the Nine Years War.
In March 1694, he was appointed Major in a new regiment raised by Colonel John Gibson, a former colleague in the Dutch Brigade.

In June 1696, a French naval force captured the British town of St. John's, Newfoundland; this was strategically important due to its proximity to the cod fishing areas of the Grand Banks.
Gibson was given command of an expedition to retake it, which included Thomas; when they arrived in early 1697, the French had already evacuated the town but destroyed it first.
Due to shortage of provisions, the majority of the force went back to Europe, leaving 300 men under Handasyd and the engineer Michael Richards to rebuild the town.
By the time Handasyd returned to England in 1698, 214 of his men had died of malnutrition or disease, testimony to the harsh conditions.
     
After the September 1697 Treaty of Ryswick ended the war, Parliament was determined to reduce costs; Gibson's Regiment was disbanded and by 1699, the English military was less than 7,000 men.
When the War of the Spanish Succession began in 1701, Thomas became Lieutenant-Colonel in the 22nd Foot, which was posted to Jamaica, a notoriously unhealthy location.
Its Colonel William Selwyn died soon after arriving in April 1702 and Thomas took over as Colonel.

As the senior military officer, Thomas also became Governor, an important position due to Jamaica's hugely profitable sugar plantations.
It was also used as a resupply point for the Royal Navy; in March 1703, an English squadron under John Graydon was sent to attack the French town of Placentia, in Newfoundland.
His ships arrived in Jamaica short of men and in poor condition; Handasyd made strenuous efforts to resupply him but the local merchants later complained about the impressment of local seamen.

His wife Anna died in September 1704 and was buried in St. Jago de la Vega or Spanish Town Cathedral.
Like his predecessor, Sir William Beeston, Thomas had a difficult relationship with the planter-dominated Jamaican Assembly.
This came to a head in 1710, when he tried to dissolve the Assembly and in the ensuing commotion, its President Peter Beckford died after allegedly falling down the stairs.

Thomas now requested he be relieved and was replaced as Governor by Lord Archibald Hamilton, while his son Roger became Colonel of the regiment.
He was promoted Major-General and returned to England in early 1711; he purchased the estate of Gaynes Hall near Great Staughton, Cambridgeshire, where he died on 26 March 1729 and was buried in the parish church of St Andrews, where his memorial still exists.
The 1638-1651 Wars of the Three Kingdoms created strong resistance in Scotland and England to a permanent military; this meant those who wanted to pursue a military career often did so in foreign armies. When the Third Anglo-Dutch War began in 1672, the Duke of Buckingham was authorised to recruit a regiment for service against the Dutch. Thomas' cousin James was a lieutenant in this unit and it is probable Thomas served with him. 
When England and the Netherlands made peace in the January 1674 Treaty of Westminster, these recruits enrolled in the Anglo-Scots Brigade, a mercenary unit in Dutch service. The first confirmed record of Thomas' service is his commission as captain in 1688 but he is referred to in 1698 as having '24 years service;' this makes it likely he saw action during the 1672-1678 Franco-Dutch War. His movements can be traced by the birth of his first two children in the Dutch port of Heusden between 1687 to 1689, while the others were born in Ireland between 1692 to 1694. 
The Brigade accompanied William III to England in the November 1688 Glorious Revolution; Thomas served in the 1689-1691 Williamite War in Ireland, including the Battle of the Boyne. After the 1691 Treaty of Limerick, he was among those transferred to Flanders during the Nine Years War. In March 1694, he was appointed Major in a new regiment raised by Colonel John Gibson, a former colleague in the Dutch Brigade. 
In June 1696, a French naval force captured the British town of St. John's, Newfoundland; this was strategically important due to its proximity to the cod fishing areas of the Grand Banks. Gibson was given command of an expedition to retake it, which included Thomas; when they arrived in early 1697, the French had already evacuated the town but destroyed it first. Due to shortage of provisions, the majority of the force went back to Europe, leaving 300 men under Handasyd and the engineer Michael Richards to rebuild the town. By the time Handasyd returned to England in 1698, 214 of his men had died of malnutrition or disease, testimony to the harsh conditions.      
After the September 1697 Treaty of Ryswick ended the war, Parliament was determined to reduce costs; Gibson's Regiment was disbanded and by 1699, the English military was less than 7,000 men. When the War of the Spanish Succession began in 1701, Thomas became Lieutenant-Colonel in the 22nd Foot, which was posted to Jamaica, a notoriously unhealthy location. Its Colonel William Selwyn died soon after arriving in April 1702 and Thomas took over as Colonel. 
As the senior military officer, Thomas also became Governor, an important position due to Jamaica's hugely profitable sugar plantations. It was also used as a resupply point for the Royal Navy; in March 1703, an English squadron under John Graydon was sent to attack the French town of Placentia, in Newfoundland. His ships arrived in Jamaica short of men and in poor condition; Handasyd made strenuous efforts to resupply him but the local merchants later complained about the impressment of local seamen.
His wife Anna died in September 1704 and was buried in St. Jago de la Vega or Spanish Town Cathedral. Like his predecessor, Sir William Beeston, Thomas had a difficult relationship with the planter-dominated Jamaican Assembly. This came to a head in 1710, when he tried to dissolve the Assembly and in the ensuing commotion, its President Peter Beckford died after allegedly falling down the stairs.
Thomas now requested he be relieved and was replaced as Governor by Lord Archibald Hamilton, while his son Roger became Colonel of the regiment. He was promoted Major-General and returned to England in early 1711; he purchased the estate of Gaynes Hall near Great Staughton, Cambridgeshire, where he died on 26 March 1729 and was buried in the parish church of St Andrews, where his memorial still exists.
Thomas Handasyd