Walden was drafted by the Angels in the 12th round of the 2006 Major League Baseball Draft, and began his professional career the next season. Walden pitched for the Orem Owlz in 2007, going 1–1 with a 3.08 ERA in 15 starts. In 64 innings, he allowed 49 hits and three home runs while striking out 63 batters. Prior to the 2008 season, Baseball America named Walden the 81st best prospect in minor league baseball. He split that season between the Cedar Rapids Kernels and the Rancho Cucamonga Quakes, going a combined 9–8 with a 2.76 ERA in 27 starts. He pitched 156 innings, allowing 122 hits and seven home runs while striking out 141 batters. Before the 2009 season, Walden was named the 70th best prospect in baseball. He pitched for the Arkansas Travelers and went 1–5 with a 5.25 ERA in 13 starts that season. In 2010, he converted to relief pitching, spending 38 games with the Travelers and six games with the Salt Lake City Bees without making a start. Walden was the California League Pitcher Of The Week for the week of August 11, 2008 with the Rancho Cucamonga Quakes. He also was named Texas League Mid-season All-Star from the Arkansas Travelers in 2010.
On November 30, 2012, the Angels traded Walden to the Atlanta Braves for pitcher Tommy Hanson.
St. Louis Cardinals
On November 17, 2014, the Braves traded Walden and Jason Heyward to the St. Louis Cardinals for Shelby Miller and Tyrell Jenkins. Walden and the Cardinals signed a two-year extension on December 23, 2014, worth $6.6 million. Through April 29, 2015, he made 12 appearances with 12 strikeouts and one run allowed in IP for a 0.87 ERA. However, he was unable to pitch after that due to shoulder stiffness. On May 6, he was diagnosed with a right shoulder strain. The Cardinals initially estimated a timetable of six to ten weeks before he could return to pitching, but recovery took much longer than expected. Instead, his shoulder did not recover, leaving the possibility of surgery to repair the rotator cuff. On September 6, the Cardinals ruled out his return for the remainder of the season. Walden rehabilitated the shoulder the following off-season in the Dallas–Fort Worth metroplex, adopting a program that was five days a week, four hours per day. He worked with former teammate Matt Belisle. After sustaining a grade 2latissimus dorsi muscle strain, the Cardinals placed Walden back on the DL to start the 2016 season.
Second stint with the Atlanta Braves
On November 12, 2016, Walden signed a minor league deal with the Atlanta Braves. The Braves and Walden mutually agreed to void his contract on February 9, 2017, after discovering that his rehab had not been progressing as expected.
Pitching style
Walden is a hard thrower, with a four-seam fastball ranging from 96 to 99 mph. His off-speed pitch to right-handed hitters is a slider in the mid 80s. He also throws the slider to lefties, but in 2011 he developed a changeup at 86–89 mph that he uses more often. His off-speed pitches work extremely well in tandem with his fastball, with whiff rates of about 48% on his changeup and 61% on his slider — each well above the league averages. He has also experimented with a two-seam fastball. The high whiff rates have contributed to a 10.9 K/9 ratio through mid-2012. Walden has an unusual pitching motion. He hops in the middle of his windup, and actually has both feet in the air for a split second, before returning to the ground and throwing the ball.
