His father was engaged in the mercantile business and was the first English speaking merchant in Detroit. In 1819 Stephen joined his father in the business in Detroit.
Shortly after that, he joined a government expedition to Great Lakes. He came into contact with some fur traders where he learned of good trading opportunities in Illinois. He worked for a fur trader named LeSallier at his trading post by the Rock River in Olgle County.
While he was there he met a Winnebago (Hochunk) woman Hononegah (Xųnųnika), the daughter of "Blacksmith." Although his wife Hinuga was Winnebago, Blacksmith was part Potawatomi.  In 1829, Stephen and Hononegah married in accord with the Indian custom, but concerns for the legal standing of their children as heirs led them in 1838 to take out a marriage license, and to finally have a formal Christian wedding in 1840. 
In 1835, after the Black Hawk War, Mack founded the town of Pecatonica where the Pecatonica River flows into the Rock River in Winnebago County. It came to be known as Macktown.
In 1841 he received his first land patent of about 80 acres. 
In 1845 he tried to interest Beloit College in establishing a school in his settlement. They chose Rockford instead. He still established a school in the second story of a house. He insisted girls, as well as boys, be allowed to attend. He also opened the school to Winnebago children.
In 1847 Hononegah died. Stephen wrote to his sister:
"Her funeral proved that I am not the only sufferer by her loss. My house is large, but it was filled to overflowing by mourning friends who assembled to pay the last sad duties to her who had set them the example how to live and how to die." Years later William C. Blinn related that after Hononegah's funeral, "a little knot of neighbors were speaking of the loss. George Stevens, the postmaster, one of the parties, said most impressively, 'The best woman in Winnebago County died last night', the neighbors all nodding in agreement." 
In 1848 he married a widow, Mrs. Isabella Daniels 
According to H. F. Kett's editors, the marriage was not a happy one: "She robbed him whenever occasion offered, and finally set fire to their house while she was under the influence of opiates, the use of which she was a great slave."
Stephen passed away in 1850 of heart disease. 
He was buried with his first wife Hononegah and a son near the Macktown golf course clubhouse. This site is presently marked by a boulder and commemorative plaque. By 1880 the cemetery site was in such bad condition that the remains of Stephen, Hononegah and their son were exhumed and moved to Phillips Cemetery in rural Rockton Township until when they were once again moved to their present location near their house in Macktown Forest Preserve. 
↑ On 5 June 1838 a marriage license was granted to Stephen Mack and Xųnųnįka that reads, "To any authorized Minister of the Gospel or Justice of the Peace in Winnebago County[,] You are hereby authorized to unite in marriage Stephen Mack and [Illegible] Hou.ne.gok. and for so doing this shall be your Warrant." ("Illinois, County Marriages, 1810-1940," database, FamilySearch (17 October 2017), citing Winnebago, Illinois, United States, county offices, Illinois; FHL microfilm 1,914,066). This was a marriage license, but the marriage itself did not take place for over two years afterwards. The Illinois Statewide Marriage Index from the Illinois State Archives records the following line: "Groom: MACK, STEPHEN, Bride: HOUNEGOK, NANCY, Date: 1840-09-14, Vol. I , County: WINNEBAGO." This is fleshed out by Bishop & Campbell (35b): “As a result of his wish to assure his children that they would be legal heirs to his property, Mack and Hononegah had another marriage ceremony performed on September 14, 1840, by Justice of the Peace William Hulin.”
"U.S. Dept. of the Interior, Bureau of Land Management, General Land Office Patents", Accession #: IL4410__.071. State: Illinois, Document Type: State Volume Patent, Issue Date: 11/10/1841, Name on Document: MACK, STEPHEN Bureau of Land Management
↑ "Illinois, County Marriages, 1810-1940," database, FamilySearch (https://familysearch.org/ark:/61903/1:1:QK9L-5V15 : 17 October 2017), Stephen Mack and Isabella Daniels, 24 Feb 1848; citing Winnebago, Illinois, United States, county offices, Illinois; FHL microfilm 1,914,065.
↑ "Illinois Mortality Schedules, 1850-1880," database with images, FamilySearch (https://familysearch.org/ark:/61903/1:1:QL91-K3XC : 16 March 2018), Stephen Mack, death Apr 1850, Illinois, United States; citing Pub. T1133 Nonpopulation Census Schedules for Illinois, 1850-1880., Film 58, NARA microfilm publications T1133 (Washington, D.C.: National Archives and Records Administration, n.d.).; FamilySearch digital folder 007283672.
↑ Accessed on ancestry.com, November 2019.
View free Ancestry Sharing Image. National Archives and Records Administration (NARA); Washington, D.C.; Non-population Census Schedules for Illinois, 1850-1880; Archive Collection: T1133; Archive Roll Number: 58; Census Year: 1850; Census Place: Rockton, Winnebago, Illinois; Page: 939.
Find A Grave: Memorial #43472999 Find A Grave, database and images (https://www.findagrave.com : accessed 29 November 2019), memorial page for Stephen Mack, Jr (2 Feb 1798–10 Apr 1850), Find A Grave Memorial no. 43472999, citing Macktown Forest Preserve, Rockton, Winnebago County, Illinois, USA ; Maintained by Dean McMakin (contributor 46804161) .
"United States Census, 1800" database with images, FamilySearch (https://familysearch.org/ark:/61903/1:1:XHRQ-GLV : accessed 20 October 2018), Stephen Mack, Tunbridge, Orange, Vermont, United States; citing p. 599, NARA microfilm publication M32, (Washington D.C.: National Archives and Records Administration, n.d.), roll 51; FHL microfilm 218,688.