French side Racing 92 announced last month that former England captain Farrell will join them at the end of the season on a two-year deal.
Moving to Paris will make the fly-half ineligible for England selection as the Rugby Football Union prevent players at foreign clubs being chosen for international duty.
Farrell, England's record points-scorer and a veteran of 112 Red Rose Tests, had previously announced he would miss this season's Six Nations in order to prioritise the mental well-being of both himself and his family.
That followed a difficult time for Farrell at last year's World Cup, where was booed by his own fans and subjected to social media abuse as England finished third at the showpiece event.
Saracens, with whom Farrell has won six English Premiership titles and three European Champions Cup crowns, said Wednesday the "most important thing" was for the playmaker to put himself and his family's well-being first.
"It's tough leaving a place that has been my home since I was 14," Farrell said in an interview published on Saracens' account on X, formerly Twitter.
"It has been a difficult decision but it is an exciting one for our family. It has never been something we have thought about before, doing anything different.
"But for the first time ever we thought it would be good to go and experience something different and the time feels right."
Farrell, the son of former England dual-code international turned Ireland coach Andy Farrell, added: "I was nervous about having some conversations, even bringing it up, because it's never been thought about, let alone discussed before.
"But once we got into the conversations they were easy to have because of the calibre of the people here.
"The club have been fantastic with us throughout this year and it really counts for something when they care about nothing but the person."
Farrell, set to make his 250th appearance for the club next month, said the aim now was to finish his time with Saracens in style.
"I want to enjoy the rest of this year. It has never been taken for granted playing for this club. People stay here for a long time, as a lot of us have," he said.
"Going in to the back end of the year we are looking to make it the best we can. I can't wait to enjoy that over the coming months."
