The Cubs said Saturday that Lester had joined them. They are scheduled to introduce him at a news conference Monday at Spiaggia Restaurant, a favorite of President Barack Obama's family.
Lester agreed Tuesday night to a $155 million, six-year contract with the Cubs. His contract contains an option for 2021 that, if it becomes guaranteed, would make the deal worth $170 million over seven seasons.
A three-time All-Star who turns 31 next month, Lester won two World Series titles with Boston. He was dealt by the Red Sox to Oakland at the trade deadline in July and helped the A's reach the playoffs.
Lester went 16-11 with a career-best 2.46 ERA and 220 strikeouts last season. He is 116-67 with a 3.58 ERA in nine big league seasons.