He is known for helping chart the northern coasts of Canada as well as his mysterious death by violence while traveling near the Turtle River in what was then part of the Territory of Iowa (now the U.S. state of North Dakota).
[2] George Simpson offered Thomas a position in the Hudson's Bay Company (HBC) in 1826, which he declined in order to complete his studies.
He enrolled in a divinity class that winter with the goal of becoming a clergyman when the offer of a position in the HBC was again extended; this time he accepted.
Simpson was stationed at the Red River Colony in the 1830s, serving as second officer to chief factor Alexander Christie.
Several writers present Simpson as an ambitious and over-confident young man, whereas Dease was twenty years older, experienced in Arctic exploration and efficient but perhaps under-confident.
They were to descend the Mackenzie River to the Arctic Ocean, turn west and close the gap between John Franklin's 1826 furthest-west and Frederick William Beechey's furthest-east at Point Barrow.
There they detached four men to go upriver to the Great Bear Lake and build winter quarters at Fort Confidence while the rest went down the Mackenzie to the Arctic, which they reached on 9 July.
They then traveled west along the coast past Franklin's Return Reef until they were blocked by ice at Boat Extreme, about 50 miles (80 km) east of Point Barrow.
Dease stayed behind with the boats and Simpson walked about 100 miles (160 km) east to a place he called Point Alexander.
The remaining problems were the possibility of a water route from Chantrey Inlet to the Gulf of Boothia and the huge rectangular area north of the coast and south of the Parry Channel.
The party returned to the Great Slave Lake in September of that year, and from there Simpson drew up a letter to the directors of the HBC describing the results of the expedition, which was published in many newspapers of the day.
To attend to preparations for this new expedition, Simpson immediately left for the Red River Colony, making the entire 1,910-mile (3,070 km) journey in sixty-one days, arriving on 2 February 1840.
On 14 June 1840, Simpson, Bird and Legros Sr were fatally shot at a wilderness camp near the Turtle River in the Territory of Iowa (now the U.S. state of North Dakota).
He had also been awarded the Royal Geographical Society's Founder's Medal, and the British government had announced its intention of granting him a pension of £100 a year.
Instead, being accused of murder and suicide, and being disgraced in the eyes of the church, Simpson was buried in an unmarked grave in Canada.
[13] Famed explorer and historian Vilhjalmur Stefansson included the Simpson case in his 1938 book Unsolved Mysteries of the Arctic.