The Grove-based marque officially hit the milestone in Germany seven days later rather than with Sunday's British Grand Prix.
But due to the fact the team are on home soil, deputy team principal Claire Williams believes they are best served by recognising such an illustrious achievement at Silverstone.
"It's not our 600th entry, that's in Germany, but we wanted to do something for our home race because that's where we are based," explained Williams to Press Association Sport.
"You know what it's like with these kind of things, there are always differing opinions as to the actual numbers.
"But we did some diligence internally and Frank (Williams, owner and team principal) has decreed it (the 600th race) is Germany.
"For some reason there is a race back in 1982 that wasn't an official entry, or something like that.
"But it doesn't matter, it's a milestone. Reaching 600 grands prix is amazing and we'll celebrate it in style, within reason of course.
"No other team, other than Ferrari and McLaren, have achieved it, so we are rightfully proud of it."
The downside for Williams is they go into the race with the unwanted record of failing to score a point in the first seven grands prix of a season for the first time in their history.
Williams added: "The most important thing at the moment is improving our current car rather than reflecting on the past.
"But for everyone who has worked at Williams over those 600 races you have to mark it. It's a massive achievement.
"And for all our employees and fans it is lovely to celebrate it this weekend.
"So to have a really good result, to score some points, for everybody involved with our team would be really great."
Given Williams were virtually unbeatable over a six-year period from 1992 to 1997, those glory days are fast fading.
But the optimist in Williams is convinced success will return as she said: "We're already looking forward to the next 600 races and another 16 world titles - or more!"