Williams was drafted in the fifth round of the 1952 NFL draft by the Chicago Bears, where he played from 1952 to 1963 and was part of the Bears' 1963 NFL Championship team. He played defensive end and tackle with sporadic appearances at linebacker, a versatility that Chicago Tribune writer Cooper Rollow suggested led to his lack of accolades as he was never named All-Pro. Nicknamed "Fat Freddy", he was popular among fans and media for his humor, frequently joking about various team-related situations; in 1959, when asked if the Bears' defensive line was the best in the league, he remarked, "I don't know if we're the best, but we sure are the ugliest."[3]
In 1964, he was traded to the Washington Redskins, along with Angelo Coia, for the Redskins' sixth overall pick in the 1965 NFL draft, which they used to select Steve DeLong.[2] When the Redskins played Williams' former team that year, he was involved in a fight with Bears center Mike Pyle after Williams jumped offside and hit Pyle in the head with his forearm.[4] Williams spent two seasons in Washington.[3]