During his football career, he earned the nickname "the Hammer" for his aggressive playing style, which incorporated martial arts techniques.

He starred as Tommy Gibbs in the 1973 crime drama film Black Caesar and its sequel Hell Up in Harlem.

[4] After high school, Williamson left Gary to attend Northwestern University[9] on a track and field scholarship,[4] earning a degree in architecture.

[10] An avid martial artist, Williamson holds black belts in Kenpō, Shotokan karate and taekwondo.

After playing college football for Northwestern[9] in the late 1950s, Williamson was signed as an undrafted free agent by the San Francisco 49ers.

[1] During his period of playing for the Chiefs, Williamson became one of football's first self-promoters, nurturing the nickname "The Hammer" because he used his forearm to deliver karate-style blows to the heads of opposing players, especially wide receivers.

Before Super Bowl I, Williamson garnered national headlines by boasting that he would knock the Green Bay Packers starting receivers, Carroll Dale and Boyd Dowler, out of the game.

[12] His prediction turned out to be an ironic one because "they (Green Bay) broke the hammer" as Williamson himself was knocked out of the game in the fourth quarter on the way to a 35–10 defeat.

He played the role of an African-American gangster in the film Black Caesar (1973) and its subsequent sequel, Hell Up in Harlem (also 1973).

During the mid-to-late 1980s and early 1990s, Williamson frequently appeared on television as a spokesman for King Cobra malt liquor ("Don't let the smooth taste fool you.

In 1994, Williamson, along with many other black actors from the 'Blaxploitation' movie era (namely Antonio Fargas, Pam Grier, Rudy Ray Moore, and Ron O'Neal) made a cameo appearance on Snoop Doggy Dogg's music video "Doggy Dogg World", where he appears as himself using his pro-football nickname "The Hammer".

Williamson co-starred with George Clooney and Quentin Tarantino in From Dusk till Dawn (1996), directed by Robert Rodriguez.

Williamson has continued his career as an actor and director into the 21st Century, appearing in the reboot film Starsky & Hutch (2004) derived from the 1970s television series.

[24] In June 2020, The Daily Beast reported that Williamson had allegedly attempted to grope an assistant costume designer during a wardrobe fitting.