At seventeen he moved to Hart, Michigan, where for a few years he worked as a salesclerk.
Then he left for Reading, Pennsylvania, where he entered into a partnership with his first cousins.
He later donated the Knox Memorial Central School Building (dedicated on July 30, 1913) that served the town until the Knox Memorial School and Edwards Central School merged.
 He initially became a partner with the Woolworths by jointly opening a Reading, Pennsylvania, Woolworth & Knox store with them on September 20, 1884, using his entire life savings.
 The Reading store's first several hours had no sales.
  However, after the partners took a lunchtime walk, they returned at 1:30 to find the local factory workers had been let out at 1:00—with their paychecks.
Sales were brisk, and the partners never looked back.
 His second store, in Newark, New Jersey, was short lived, but his partnership thrived nonetheless.
 The third venture, in Erie, Pennsylvania, enabled them to buy out the Newark lease.
 He partnered with Frank to open the first Buffalo store, at 409 Main Street, on October 13, 1888.

By 1889, he was able to buy out his cousins.
He maintained a collegial business relationship with his cousins after the buyout.
 In fact, he bought Woolworth merchandise at wholesale and sometimes traded in competition.
 He formed another brief partnership with another friendly rival, Earle Perry Charlton, from 1889–1895, opening his Buffalo "S.H. Knox Co." 5 and 10 Cent Store in 1890.
 In 1890, he made established headquarters in the Buffalo store.
Sources disagree on the chronology of later stores.
 One source says that the second Buffalo store was opened at 549 William Street on June 20, 1891.
Another says Knox opened his second store on December 18, 1893, at 519 Main Street four days after the first store at 409 Main Street was destroyed in the Wonderland Building Fire.
 The 519 Main Street store replaced the 409 Main Street after the December 14, 1893 fire and moved to 395 Main Street in 1895.
He continued to build his S.H. Knox Co. 5 and 10 Cent Store empire.
By the time of the 1911 incorporation of F. W. Woolworth Company, Knox was the second largest of six store operators with 98 U.S. and 13 Canadian locations.
In 1912, he received $12 million of the $65 million merger proceeds and was appointed Director and Senior Vice-Principal of the Corporation.
 Knox is remembered as the pioneering city center store operator.
His Detroit, Michigan store was the first outside of the agricultural and small-market towns.
Many of the Woolworth friendly rivals emulated his plan.

In 1913, he purchased Stephen Clement's interests in Marine National.
  At his death, Seymour was Vice President of the Woolworth Co. and Chairman of the Board of the Marine Trust Co.  He was the first of three generations of the family to serve as Chairman.
At seventeen he moved to Hart, Michigan, where for a few years he worked as a salesclerk. Then he left for Reading, Pennsylvania, where he entered into a partnership with his first cousins. He later donated the Knox Memorial Central School Building (dedicated on July 30, 1913) that served the town until the Knox Memorial School and Edwards Central School merged.  He initially became a partner with the Woolworths by jointly opening a Reading, Pennsylvania, Woolworth & Knox store with them on September 20, 1884, using his entire life savings.  The Reading store's first several hours had no sales.   However, after the partners took a lunchtime walk, they returned at 1:30 to find the local factory workers had been let out at 1:00—with their paychecks. Sales were brisk, and the partners never looked back.  His second store, in Newark, New Jersey, was short lived, but his partnership thrived nonetheless.  The third venture, in Erie, Pennsylvania, enabled them to buy out the Newark lease.  He partnered with Frank to open the first Buffalo store, at 409 Main Street, on October 13, 1888.
By 1889, he was able to buy out his cousins. He maintained a collegial business relationship with his cousins after the buyout.  In fact, he bought Woolworth merchandise at wholesale and sometimes traded in competition.  He formed another brief partnership with another friendly rival, Earle Perry Charlton, from 1889–1895, opening his Buffalo "S.H. Knox Co." 5 and 10 Cent Store in 1890.  In 1890, he made established headquarters in the Buffalo store. Sources disagree on the chronology of later stores.  One source says that the second Buffalo store was opened at 549 William Street on June 20, 1891. Another says Knox opened his second store on December 18, 1893, at 519 Main Street four days after the first store at 409 Main Street was destroyed in the Wonderland Building Fire.  The 519 Main Street store replaced the 409 Main Street after the December 14, 1893 fire and moved to 395 Main Street in 1895. He continued to build his S.H. Knox Co. 5 and 10 Cent Store empire. By the time of the 1911 incorporation of F. W. Woolworth Company, Knox was the second largest of six store operators with 98 U.S. and 13 Canadian locations. In 1912, he received $12 million of the $65 million merger proceeds and was appointed Director and Senior Vice-Principal of the Corporation.  Knox is remembered as the pioneering city center store operator. His Detroit, Michigan store was the first outside of the agricultural and small-market towns. Many of the Woolworth friendly rivals emulated his plan.
In 1913, he purchased Stephen Clement's interests in Marine National.   At his death, Seymour was Vice President of the Woolworth Co. and Chairman of the Board of the Marine Trust Co.  He was the first of three generations of the family to serve as Chairman.
Seymour H. Knox I