Born in Fort Wayne, Indiana, his full name is William Ralph Blass. His father owned a small hardware store in Fort Wayne but he committed suicide when Bill Blass was five years old. Bill Blass went to South Side High School at Fort Wayne. By the age of 15 he was having his drawings featured in local newspapers, and he was selling designs to Seventh Avenue manufacturers. He won second prize in a fashion contest sponsored by The Chicago Tribune. In 1939 he moved to New York, aged 17, to study fashion design at Parsons School of Design

Blass then took a job as a sketcher for the sportswear firm David Crystal. During World War II he was assigned to the specialised counter-intelligence unit, 603rd Camouflage Battalion. In 1944-1945 he joined the invasion of Europe through the Battle of the Bulge and crossing the Rhine.

When he returned to New York he first worked for Anne Klein. He then worked as an anonymous designer for the ready-to-wear firm Anna Miller Company. When Anna Miller retired in 1959 Bill Blass had become her head designer and he remained with the business when her brother Maurice Rentner merged their companies and so taking on Bill Blass into his Seventh Avenue company. Within two years Bill Blass was appointed vice-president. After establishing his reputation in women's couture he branched out into men's fashion in 1967.

In 1960 Maurice Rentner died and the labels began to have 'Bill Blass for Maurice Rentner'. In 1970 Bill Blass bought the company and in 1973 he renamed it Bill Blass Ltd. He produced clothes for many renowned women including Jacqueline Kennedy, Nancy Reagan, Barbara Bush, Pat Buckley, Brook Astor, Nancy Kissinger, Happy Rockefeller, Gloria Vanderbilt, Jessye Norman, the singer Barbra Streisand, and the TV news star Barbara Walters.

In 1977 he was awarded an honorary doctorate by the Rhode Island School of Design. In 1978 he launched a signature perfume, and he signed deals for sales lines in menswear, confectionery, and bed linen, eventually building up to 97 different licensing agreements. For several years he signed an annual Bill Blass Lincoln limousine. In 1981 he was given the Gentleman's Quarterly Award. In 1987 he was appointed by President Ronald Reagan to the President's Committee on the Arts and Humanities.

In 1994 he donated $10 million to the New York Public Library which used the money to set up a public reading room in his name. In 1999 he received the first lifetime achievement award from the Fashion Institute of Technology. He won the Coty American Fashion Critics award three times. Also in 1999 he sold his business to his former chief finance officer for a reported $50 million. It had an annual turnover exceeding $700 million.

He was one of the founder members of the Council of Fashion Designers of America (CFDA). He was the first to receive the CFDA Perignon Award for Humanitarian leadership beyond fashion. He donated the $25000 prize to the AIDS care centre of New York Hospital. He was also a major donor to Gay Men's Health Crisis at a time when well-known people were silent about AIDS.

He died in New Preston, Connecticut, of cancer aged 79. There is a bronze marker in Bill Blass's name in the fashion Walk of Fame on Seventh Avenue, New York.