On March 8, Lester agreed to a five-year, $30 million contract extension with a $14 million team option in 2014.
On May 3, 2014, Jon Lester pitched 8 one-hit innings against the Oakland Athletics, where he had a career-high 15 strikeouts. It was the most by a Red Sox pitcher since Josh Beckett in 2010.
He was elected to his third all-star game in July 2014 after posting a 2.73 ERA over his first 18 starts of the season.
On July 31, 2014, Lester and Jonny Gomes were traded to the Oakland Athletics for Yoenis Céspedes and a competitive round 2015 draft pick. The move reunited Lester with former teammates Jed Lowrie, Coco Crisp, Brandon Moss and Josh Reddick. He won his debut start against the Kansas City Royals on August 2. He had a 6–4 record and a 2.35 ERA, while pitching for the A's.
In December 2014, Lester agreed to a six-year, $155 million deal with the Chicago Cubs with a vesting option for a seventh year at $15 million.