William's career began in the 1970s when she became a backup vocalist for Stevie Wonder, who was the opening act for The Rolling Stones at the time, and Minnie Riperton. She left Wonder in 1975 and teamed up with producer Maurice White, who was the leader of Earth, Wind & Fire. After signing to Columbia Records, her 1976 debut album entitled This Is Niecy was released. The single "Free", reached number two on the Black Singles chart, and #1 on the British Singles chart. The album also featured "Cause You Love Me Baby" and "That's What Friends Are For". She also shared a #1 hit on the Billboard Hot 100 chart with pop singer Johnny Mathis in 1978 with the duet "Too Much, Too Little, Too Late". The duet also topped the Black Singles and Adult Contemporary charts.

Williams moved on to the American Recording Company (ARC) in the early 1980s where she scored the top ten smash hit "Silly." in 1981. The following year, famed producer Thom Bell helped Williams score another #1 R&B chart-topper with her remake of The Royalettes' "It's Gonna Take a Miracle." In 1984, Williams released the album "Let's Hear It for the Boy", in which the title track reached #1 on the Billboard Hot 100 and was featured on the soundtrack to the motion picture Footloose.

Williams continued releasing during the 1980s such as Hot On The Trail (1986), Water Under The Bridge (which featured her final hit song 'Never Say Never') in (1987), and As Good As It Gets (1988). However, in part due to a lack of promotion from her record company, her mainstream popularity faded.