The pair – who have fallen out of England contention this year – had been linked with lucrative deals in France, with Lyon tipped as one possible destination.
Saracens Director of Rugby Mark McCall said of the signing: “Everyone involved with Saracens will be thrilled that Mako and Billy have chosen to extend their time at the club. In their own unique ways, they have been instrumental in how we have grown and progressed and have been key figures in the success the club has enjoyed.
“Mako is a world class operator, with an incredible skill set and an innate understanding of the game. His competitive spirit and leadership have helped drive standards within the group and he has the desire and determination to take his own game and that of the team to new levels in the years ahead.
“Billy’s level of performance for nearly a decade at the club has been staggering and he has demonstrated great resilience and perseverance to overcome a number of serious injury setbacks over that time. He is highly influential on and off the field and has a great feel for what the group needs at any given moment.
“We are grateful to Mako and Billy for the loyalty they have shown to the club.”
Billy, who has played 135 times for Saracens, has won four Premiership titles and three European titles during his stint at StoneX. The 29-year-old also has 61 England caps to his name, firmly establishing him as one of the most feared number eights in the game.