Kelley opened the 2019 season back with Pawtucket,[2] where through the end of June he had a 5–2 record in 28 games (all in relief) with a 0.96 ERA and 34 strikeouts in 37+2⁄3 innings.[5] On July 1, the Red Sox announced that they would add Kelley to their active major league roster;[6][7] he made his MLB debut on July 2, pitching the ninth inning in a 10–6 win over the Toronto Blue Jays.[8] Kelley was optioned to Pawtucket on July 4.[9] In late August, he was named to the International League post season all-star team.[10] He was recalled to Boston on September 4, following the end of the Triple-A season.[11] In mid-September, Kelley was named a Triple-A post season all-star by Baseball America,[12] and was named the recipient of the Red Sox' Lou Gorman Award.[13] Overall with the 2019 Red Sox, Kelley appeared in 10 games, compiling an 0–3 record with an 8.64 ERA and six strikeouts in 8+1⁄3 innings.
Philadelphia Phillies
On December 2, 2019, Kelley was claimed off waivers by the Philadelphia Phillies.[14] He was designated for assignment by the Phillies on January 31, 2020, and outrighted on February 5.[citation needed] Kelley made the Opening Day roster for the Phillies in 2020.[15] He was designated for assignment again by the Phillies on August 11. He elected free agency on September 28.
Chicago Cubs
On December 21, 2020, Kelley signed a minor league contract with the Chicago Cubs.[16] On April 24, 2021, Kelley was released by the Cubs organization.[17]
Atlanta Braves
On April 28, 2021, Kelley signed a minor league contract with the Atlanta Braves organization.[18]
Kelley spent the 2021 season with the Triple-A Gwinnett Stripers, making 37 appearances with a 1.52 ERA and 46 strikeouts. Kelley became a free agent following the 2021 season.
Milwaukee Brewers
On November 18, 2021, Kelley signed a minor league deal with the Milwaukee Brewers, with an invite to spring training.[19] He had his contract selected on May 17, 2022. On the year, Kelley made a career-high 18 appearances, logging a 6.08 ERA, striking out 23 in 23.2 innings of work, and collecting his first career win.
On January 4, 2023, Kelley was designated for assignment by the Brewers following the acquisition of Bryse Wilson.[20] On January 11, Kelley cleared waivers and was assigned outright to the Triple-A Nashville Sounds. However, he rejected the outright assignment and elected free agency the same day.[21]
Tampa Bay Rays
On January 18, 2023, Kelley signed a minor league contract with the Tampa Bay Rays organization.[22] On February 16, Kelley was selected to the 40-man roster,[23] but the move was voided the following day as, as a non-roster invitee, Kelley was ineligible for the 40-man until March 15.[24] On March 15, the Rays officially selected Kelley to the 40-man roster, placing Andrew Kittredge on the 60-day injured list to clear roster space.[25] He was optioned to the Triple-A Durham Bulls to begin the 2023 season.[26]
On May 12, 2023, Kelley made his first major league start against the New York Yankees.[27] In 10 games for Tampa Bay, he registered a 5.87 ERA with 11 strikeouts in 15+1⁄3 innings of work. On September 22, Kelley was designated for assignment following the promotion of Junior Caminero.[28] He cleared waivers and was sent outright to Durham on September 24.[29] Kelley elected free agency following the season on November 6.[30]
Seattle Mariners
On February 23, 2024, Kelley signed a minor league contract with the Seattle Mariners.[31]