The Briton joined the outfit as part of its driver academy during last season and as well as some simulator work she attended several European grands prix.
For 2020, she will continue working with the outfit at tracks and at the factory, and will increase her simulator programme to help with the team's development.
Speaking about her new deal, Chadwick said: "The time spent in the simulator has been invaluable and I have enjoyed every moment, immersing myself within the team. I look forward to continuing to work with Williams this year."
Deputy team principal Claire Williams added: "I have thoroughly enjoyed watching Jamie develop, and excel, both with the team and her time racing in W Series.
"Jamie has done an excellent job promoting women in motorsport and we are extremely proud of the work she has done."
As well as her W Series title success, Chadwick won the 2015 British GT GT4 title, was a British F3 race winner in 2018 and won the MRF Winter Series in 2018/2019.
She is continuing in W Series this year, and is also taking part in the Asian F3 championship.