Rob Smedley, Williams' head of vehicle performance, will depart the team at the end of the year.
This year, however, they are stuck at the foot of the table with a deficit of 26 points to Scuderia Toro Rosso and only two races remaining.
Smedley, who started in Formula 1 with Jordan at the turn of the century, has not given any indication of where he may land next.
“I have thoroughly enjoyed my time at Williams,” said the Brit.
“The team has been through great change since I joined in 2014 and it has been a pleasure to have played a part in that.
“Williams is a very special team within the F1 community and I'm certain that with all the talent that we have here they will go on and do better things. The team will always remain close to my heart.
“After 20 years in Formula One, however, I feel it's the right time to reflect on things and evaluate what the next move is.
“I'm thoroughly looking forward to spending more time with my family before deciding on future opportunities.”
Deputy Team Principal Claire Williams added, “It has been a pleasure having Rob in our team for the last four years and we will miss him both personally and professionally.
“He agreed to join us at a time when our performance was low, and we are grateful that he saw the potential for us to turn things around at that time.
“During his time with us, he has made a significant contribution to the team through his role, but he's also been just a great person to have at Grove, inspiring many with his enthusiasm and passion for Formula One.
“Rob's been in the sport a long time, so we totally understand and respect his decision to take some well-deserved time out to spend with his wife and boys.
“We wish him all the best and thank him wholeheartedly for the time, passion and fighting spirit he's given to Williams.”