Senna's 28-year-old nephew, Bruno, signed a one-year deal to partner Venezuelan driver Pastor Maldonado and give the British team an all-South American lineup going into the March 18 opener in Australia.
While Bruno Senna doesn't carry the success or aura his uncle Ayrton commanded, the Sao Paulo native does provide one of the most valuable names in F1 to a team looking to regain its lustre after one of its worst seasons.
"This is very much a family business in our case and everyone is super happy about it," a "thrilled" Senna told reporters on a conference call on Tuesday, when he suggested the pressure was already on.
"Last year was the worst of their history and they don't want a repeat of that. It is very important for them to have a very good year this year, and the pressure is on everybody."
Ayrton Senna joined Williams from McLaren in 1994 and was fatally wounded during a crash at San Marino, only his third race with the team. Bruno used to race karts with his uncle on the family farm as a child.
"It will be very interesting to drive for a team that my uncle has driven for, particularly as quite a few of the people here actually worked with Ayrton," Senna said. "Hopefully we can bring back some memories and create some great new ones, too."
Senna replaces fellow Brazilian driver Rubens Barrichello, who leaves the team after two seasons in which he picked up a combined 51 points, but only four last year. Barrichello escaped death from a serious crash during the weekend of the very same San Marino GP that took Ayrton Senna's life.
"I wish my friend (Bruno Senna) all the best ... the future is wide open," the 39-year-old former Ferrari and Brawn driver wrote on Twitter.
Senna served as a test driver for Lotus Renault last season before replacing Nick Heidfeld for the last eight races, scoring two points.
He drove in 18 GPs for Spanish team HRT in 2010. Williams did not disclose the length of Senna's deal.
"The circumstances of Bruno's two seasons in Formula One have not given him an ideal opportunity to deliver consistently so it was essential that we spent as much time with him as possible to understand and evaluate him as a driver," team principal Frank Williams said. "He has proven quick, technically insightful and above all capable of learning and applying his learning quickly and consistently. Now we are looking forward to seeing that talent in our race car."
Williams has won nine constructor championships and seven driver titles, but none since it swept the two in 1997. The team is coming off one of its worst ever seasons since its debut in 1975, scoring just five points.
Senna will get his first taste of his new drive at the season's first official test session in Jerez, Spain, with the Sao Paulo native getting behind the wheel on Feb. 9.
"It's nice to start the season at the same level as the other guys, I never had that chance before," Senna said. "That should give me a chance to be ready for all kinds of situations since I'll know the car as well as my teammate."
Senna's signing was also reportedly pushed through because of his strong sponsorship, which includes Embratel and Gillette.