Kennedy made his professional debut with the Arizona League Diamondbacks, batting .270 with nine doubles, eight triples, and twenty RBIs over fifty games.[4] In 2018, he played with the Missoula Osprey with whom he slashed .327/.396/.465 with four home runs and 32 RBIs over 57 games.[5] For the 2019 season, he played for the Kane County Cougars, hitting .262 with seven home runs and 49 RBIs over 101 games.[6] He did not play a minor league game in 2020 after the season was cancelled due to the COVID-19 pandemic.[7][8] Kennedy began the 2021 season with the Hillsboro Hops and was promoted to the Amarillo Sod Poodles in mid-June.[9][10] Over 96 games between the two clubs, Kennedy slashed .290/.384/.523 with 22 home runs, sixty RBIs, and 16 stolen bases.[11] He was selected to play in the Arizona Fall League for the Salt River Rafters after the season.[12]
Kennedy was assigned to the Reno Aces of the Triple-APacific Coast League to begin the 2022 season.[13] On June 17, the Diamondbacks selected his contract and he made his MLB debut that night as the starting designated hitter versus the Minnesota Twins.[14] He recorded his first MLB hit that night with a single off of Devin Smeltzer.[15] On June 19, he recorded his first MLB home run, a grand slam versus Caleb Thielbar of the Minnesota Twins.[16] He was sent outright off the roster on November 11, 2022.
He was assigned to Triple–A Reno to begin the 2023 season, where he played in 87 games and hit .318/.447/.480 with 5 home runs and 46 RBI. On August 12, 2023, the Diamondbacks selected Kennedy's contract, adding him to the major league roster.[17] In 10 games for Arizona, Kennedy went 4–for–24 (.167) with 1 RBI. On September 6, he was designated for assignment following the waiver claim of Seby Zavala.[18]
Oakland Athletics
On September 9, 2023, Kennedy was claimed off waivers by the Oakland Athletics.[19] In 11 games for the Triple–A Las Vegas Aviators, Kennedy hit .216/.326/.216 with no home runs, four RBI, and six walks.
Detroit Tigers
On October 25, 2023, Kennedy was claimed off waivers by the St. Louis Cardinals.[20] He was designated for assignment on February 27, 2024, following the signing of Brandon Crawford[21] and claimed off waivers by the Detroit Tigers on February 29.[22] Kennedy was optioned to the Triple–A Toledo Mud Hens to begin the 2024 season.[23] The Tigers called up Kennedy on April 20 to replace Gio Urshela, who was placed on the injured list.[24] In 6 games for Detroit, he went 2–for–10 (.200) with one home run and four RBI. On June 3, Kennedy was designated for assignment by the Tigers.[25]
Philadelphia Phillies
On June 7, 2024, Kennedy was traded to the Philadelphia Phillies in exchange for cash considerations.[26][27] On September 9th, 2024 Kennedy had his first career plate appearance with the Phillies in a 2-1 win over the Tampa Bay Rays. Fans at the stadium chanted "Buddy" and Kennedy walked.[28]