Son of Samuel Sinclair and Rebecca Barrett.
Information on William's gravestone states that he emigrated to North Carolina in 1818 and moved to Tennessee in 1834. This same information is also recorded in his family bible. His brother Gregory also came to America with him in 1818, leaving the remainder of the family to arrive in 1820 under the leadership of brother John.

When William arrived in America he went to work for Samuel Blackwood in Lincoln County, North Caroline, from whom he rapidly learned the trade of blacksmith. Both William and his brother Gregory worked hard and were diligent in their Scottish heritage. William purchased 156 acres of land on Crowders Creek, Lincoln County from Samuel Carson on 17 February 1822 which he later increased with various purchases to 200 acres. The following year William felt sufficiently secure to wed. Contrary to the common practice of marrying someone within your immediate neighborhood, William chose his bride from across the state line in York County, South Carolina. This is the first instance indicating that William might be the non-conformist in the family. His choice was Mary Harper, daughter of Matthew and Jane Harper.

Marriage records for William have not been located. The Pisgah Church records in Gaston County, North Carolina do not date back that far. Date of his first marriage is found in the William Sinclair bible. The names and dates of birth of all of his children are also found in this record. Without a doubt his first marriage took place in York County, South Carolina where his bride lived. The second probably took place in Lincoln County, North Carolina as both William and Mary M. Oates were residents of that county. The area in which they lived later became Gaston County, North Carolina.

William is found on the 1830 Lincoln County, North Carolina census (30-40) as the head of the household. At that time his household consisted of himself, a female 30-40, three young females and 2 young males. This would have been William, his wife Mary Harper Sinclair and the five children born of that marriage.

William is found on the 1836 Wayne County, Tennessee tax list. He paid tax on 346 acres of land and two slaves. 1 white poll.

He is next found on the 1840 Wayne County, Tennessee census, aged 40-50, two males 10-15, 2 females 0-5, 2 females 10-15, 1 female 15-20 and one female 30-40.

In 1850 William is found in Lauderdale County, Alabama. He is listed on this census as William St. Clair, aged 56. (family 418/430). Also in the household is his second wife Mary, the two youngest children from his first marriage and four children from the second marriage. The two youngest children list their place of birth as Alabama.

On Oct 6, 1852 William Sinclair, Sr. was appointed one of the Judges to conduct the election for President. Source: County Court Minutes Sept 1848-June 1854. Page 86, WAYNE COUNTY HISTORIAN, Volume 6, #2 (June 1993).

The property on which the cemetery is located has been in the Sinclair family for about 150 years. It is land that was originally owned by John Sinclair, brother of William.

William was the father of five children of the first marriage and thirteen children of the second marriage, making a total of eighteen children. In the second marriage, there were six children who died young and unnamed. Source is the William Sinclair bible.

There is no indication that William had a middle name or an initial.