Wolff will be relinquishing his seat on the Williams board but will retain his shareholding in the company.
Williams team principal and majority shareholder Frank Williams will continue to oversee the running of the company with Wolff's responsibilities being shared between members of the board's executive committee.
"I would like to thank Toto for his hard work, dedication and commitment to the team during his time as a director of the company," said Williams on the F1 website.
"He was a key support to me as executive director last season, deputising at a number of races when I was unable to attend. However, positions such as the one offered to him by Mercedes do not come around often. Toto has a long history with them and I certainly was not going to stand in the way of him accepting this once in a lifetime opportunity."
Austrian Wolff joined the Williams board after investing in the team in late 2009 and became an executive director in July last year. His departure follows Nobert Haug's recent exit as head of Mercedes-Benz Motorsport.