2009-10: Stephon Williams played high school hockey for Lathrop High in Fairbanks as a junior and skated for the Arctic Lions midget team. He was a second-team all-star in the Large Schools division behind Alaska high school Player of the Year Blake Mendenhall. Williams was selected by Sioux Falls in the 21st round (311th overall) of the 2010 USHL Entry Draft.
2010-11: Williams played 35 games for Sioux Falls in his first USHL season – sharing the goaltending duties for the Stampede with Finnish 20-year-old Juho Olkinuora. Williams was 20-7-6 with 1 shutout and had a 2.59 goals against and .909 save percentage. Sioux Falls finished third in the Western Division and reached the division finals. Williams was 4-1 in five playoff games; posting a 2.21 goals against and .924 save percentage. In February 2011 he committed to playing college hockey at Minnesota State in 2012-13.
2011-12: Williams began the year with Sioux Falls and was obtained by Waterloo in a February trade – backstopping the Black Hawks to the USHL Finals. Williams was 6-9-2 with the Stampede with a 2.70 goals against and .913 save percentage before the deal – which sent goalie Jay Williams to Sioux Falls. He was 10-6-2 with a 2.79 goals against and .896 in 19 regular season games for Waterloo. The Black Hawks put things together in the playoffs after finishing third in the Western Division. Williams started 15 playoff games and was 10-5 with 1 shutout. He finished with a 2.28 goals against and .922 save percentage. Waterloo took Green Bay to five games in the finals as the Gamblers won their second title in three years.
2012-13: Williams emerged as the starter for Minnesota State as a freshman – playing in 35 games for the Mavericks. He was 21-12-2 with 4 shutouts and had a 2.00 goals against and .924 save percentage. Minnesota State reached the WCHA Final Five quarterfinal and earned an at-large bid to the NCAA Tournament. Williams stopped 22 of 25 shots in a 4-0 loss to Miami in the Midwest Regional. Not among the 35 North American goalies in Central Scouting's final rankings, Williams was selected by the Islanders in the fourth round (106th overall) of the 2013 NHL Draft; one round after New York selected his former Waterloo teammate Eamon McAdam.