Ian Kennedy's journey through Major League Baseball began when he was selected by the New York Yankees in the first round of the 2006 MLB Draft. A right-handed pitcher, Kennedy quickly ascended through the Yankees' minor league system, making his MLB debut on September 1, 2007. In his first season, he showed promise, posting a 1.89 ERA in three starts, which helped him secure a spot in the Yankees' rotation for the following year.

The 2008 season, however, proved challenging for Kennedy. Struggling with consistency, he finished the year with an 8.17 ERA over 10 starts. His difficulties continued into 2009, leading to a trade to the Arizona Diamondbacks as part of a three-team deal that also involved the Detroit Tigers. This move marked a turning point in Kennedy's career.

With the Diamondbacks, Kennedy found his stride. The 2010 season saw him become a mainstay in Arizona's rotation, where he posted a 3.80 ERA over 32 starts. His breakout year came in 2011 when he won 21 games, tied for the most in the National League, and recorded a 2.88 ERA. This performance earned him a fourth-place finish in the NL Cy Young Award voting and solidified his reputation as a reliable starting pitcher.

Kennedy continued to pitch for the Diamondbacks until 2013, when he was traded to the San Diego Padres. His tenure with the Padres was marked by mixed results. While he demonstrated the ability to strike out batters, leading the National League in strikeouts per nine innings in 2014, his ERA remained above 4.00 during his time in San Diego.

In 2015, Kennedy was traded to the Kansas City Royals. The following year, he signed a five-year contract with the team, transitioning from a starter to a reliever in the latter part of his contract. His adaptability was on display in 2019 when he successfully converted to a closer role, saving 30 games for the Royals.

Kennedy's career took another turn in 2020 when he joined the Texas Rangers. Although the season was shortened due to the COVID-19 pandemic, he continued to contribute as a veteran presence in the bullpen. In 2021, he was traded to the Philadelphia Phillies, where he served as a closer and provided stability to their bullpen during the playoff race.

Throughout his career, Ian Kennedy demonstrated resilience and adaptability, navigating the challenges of being both a starter and a reliever.