Thomas Walsh/Welch was born in Philadelphia where his father had settled after emigrating from Northern Ireland in 1740. When his father was captured and imprisoned by the French, Thomas went to live with his mother's Quaker relatives who provided him with a good education. He served with the British forces during the Seven Years War, and at its conclusion completed his mathematical studies and became a surveyor in Berks County, Pennsylvania. In 1769 he moved to Frederick County, Maryland, where he was again employed as a surveyor.
With the outbreak of the American Revolution he joined the Maryland Loyalists and was commissioned as a quartermaster. His unit was sent to Pensacola, Florida, which was captured by the Spanish in May of 1781. After the prisoners were exchanged, they were sent to New York City.
At the end of the war he was placed in command of a group of Loyalist refugees who were going to New Brunswick. Here he received a commission as a land surveyor and spent several years surveying land for refugees. Five years later he decided to return to Maryland, but was shipwrecked en route and lost all his documents and possessions.
By 1793 he had become discouraged with attempts to recover his properties in the United States and came with his family to Queenston, Upper Canada, where he was appointed a surveyor in Lincoln County, a captain in the militia, and a justice of the peace. In 1796 he moved to Norfolk County and was appointed Norfolk's first land registrar. He was responsible for the survey of several townships and of the proposed town of Charlotteville on Turkey Point. In 1797 he settled in Charlotteville township where he farmed, owned a mill, was captain of the 1st Norfolk Militia, and carried out the official duties of clerk of the peace, clerk and later judge of the district court, registrar and later judge of the surrogate court, district road commissioner, deputy land registrar, and several other positions. He was succeeded in a number of his offices by his son Frances Leigh Walsh/Welch.