He began his professional career in 2010 as a member of the Williamsport Crosscutters, with whom he started 13 games, losing only one of them, and recording a 4.21 earned run average (ERA).
At the conclusion of the season, he played in the Florida Instructional League.
He progressed to the Class A and Class A-Advanced level in 2011, pitching for both the Lakewood BlueClaws and the Clearwater Threshers, and combining to post a 3.49 ERA in 26 starts.
He finished third in the South Atlantic League (with Lakewood) by posting 11 wins, and finished seventh with a 3.38 ERA.
His steady advancement through the minor league system continued in 2012; the next year, he spent a full season with the Reading Fightin Phils of Class AA.
Despite his fast ascent through the minor league system, he flew "under the radar" with Reading, despite a 3.86 ERA.
However, his season ended in June when the Phillies placed him on the disabled list due to a ligament injury in his middle finger.

After splitting his time in the 2013 season between the Fightin Phils and the Lehigh Valley IronPigs (Class AAA – International League), Buchanan attended a Prospect Education Program hosted by the Phillies organization in January 2014.
At the time, he was the only one of the eight attendees who had not been invited to major league camp at spring training, however after taking a "leadership role" at the program, Joe Jordan (the Phillies' director of player development) and manager Ryne Sandberg tendered Buchanan an invitation to big league camp.
He entered camp competing for a spot in the Phillies rotation, and impressed the Phillies' staff, ultimately becoming a finalist for the position with Jeff Manship.
He did not make the opening day roster despite his strong performance, but would remain in consideration for promotion during the 2014 season, particularly as a spot starter or even the Phillies' fifth starter if needed due to injury.
Buchanan commented, "I wanted to come in here and make an impression and open us some eyes and I feel like I did that.
I feel like I proved myself that I am ready for the next level.
I had a blast every day in camp."

With his college coach on hand to witness a "monumental time in Georgia State baseball history", Buchanan made his major league debut against the Los Angeles Dodgers on May 24, 2014.
Replacing Cliff Lee, who was injured, Buchanan pitched five innings, and struck out two batters, and allowed five hits, two runs, and no walks in only 66 pitches, en route to earning the win.

Buchanan was designated for assignment by the Phillies in November 2016, and released on November 23.
He began his professional career in 2010 as a member of the Williamsport Crosscutters, with whom he started 13 games, losing only one of them, and recording a 4.21 earned run average (ERA). At the conclusion of the season, he played in the Florida Instructional League. He progressed to the Class A and Class A-Advanced level in 2011, pitching for both the Lakewood BlueClaws and the Clearwater Threshers, and combining to post a 3.49 ERA in 26 starts. He finished third in the South Atlantic League (with Lakewood) by posting 11 wins, and finished seventh with a 3.38 ERA. His steady advancement through the minor league system continued in 2012; the next year, he spent a full season with the Reading Fightin Phils of Class AA. Despite his fast ascent through the minor league system, he flew "under the radar" with Reading, despite a 3.86 ERA. However, his season ended in June when the Phillies placed him on the disabled list due to a ligament injury in his middle finger.
After splitting his time in the 2013 season between the Fightin Phils and the Lehigh Valley IronPigs (Class AAA – International League), Buchanan attended a Prospect Education Program hosted by the Phillies organization in January 2014. At the time, he was the only one of the eight attendees who had not been invited to major league camp at spring training, however after taking a "leadership role" at the program, Joe Jordan (the Phillies' director of player development) and manager Ryne Sandberg tendered Buchanan an invitation to big league camp. He entered camp competing for a spot in the Phillies rotation, and impressed the Phillies' staff, ultimately becoming a finalist for the position with Jeff Manship. He did not make the opening day roster despite his strong performance, but would remain in consideration for promotion during the 2014 season, particularly as a spot starter or even the Phillies' fifth starter if needed due to injury. Buchanan commented, "I wanted to come in here and make an impression and open us some eyes and I feel like I did that. I feel like I proved myself that I am ready for the next level. I had a blast every day in camp."
With his college coach on hand to witness a "monumental time in Georgia State baseball history", Buchanan made his major league debut against the Los Angeles Dodgers on May 24, 2014. Replacing Cliff Lee, who was injured, Buchanan pitched five innings, and struck out two batters, and allowed five hits, two runs, and no walks in only 66 pitches, en route to earning the win.
Buchanan was designated for assignment by the Phillies in November 2016, and released on November 23.
David Buchanan (baseball)