Seibald was elected to the Sphinx Head Society, Cornell's oldest senior honor society. Seibald is a five-time Major League Lacrosse All-Star, and is fifth on the all-time leader board for two-point goals. He played for the Denver Outlaws for three years, for the New York Lizards for three years, and most recently for the Boston Cannons. He also played professionally indoor as well for the Philadelphia Wings from 2010 to 2013. Seibald was selected 2nd overall by the Denver Outlaws in the 2009 Major League Lacrosse draft. He was a 2009 MLL All-Star. On September 9, 2009, Seibald was selected 8th overall by the Boston Blazers in the indoor National Lacrosse League draft, and was traded to the Philadelphia Wings shortly into the 2010 season. In the 2010 MLL All-Star Game skills competition, Seibald posted a world record-tying shot of 110 miles per hour. He shared the record with Team USA teammate Paul Rabil, who defended his 2009 fastest shot title by posting a second shot of 111 to Seibald's 109 miles per hour second shot. He was a 2011 MLL All-Star. In 2012, Seibald set a career high in Loose Balls (77), while helping the Wings make the playoffs for the first time in 5 years. He was a 2012 MLL All-Star. Before the 2013 season, Seibald injured his foot in training camp, and played in six games in 2013. He was a 2014 and 2015 MLL All-Star. In January 2015 Seibald was traded by the New York Lizards to the Boston Cannons, and named tri-captain. In 2016 and 2017 he was team captain. Seibald announced his retirement from professional lacrosse following the conclusion of the 2018 season. Seibald played on the US Men's National Lacrosse Team at the 2010 World Lacrosse Championships in Manchester, England. The US team defeated Canada to win the World Championship. His jersey number was 42, the same number he wore at Cornell. He also played for the 2014 US Men's National Lacrosse Team at the 2014 World Lacrosse Championships in Denver, Colorado. Seibald was announced as a member of Israel's 150-man player pool for the 2018 World Lacrosse Championship. Seibald is Jewish, and is thus eligible for Israeli citizenship under Israel's Law of Return. He was named to the team, and competed on it in the World Championships in Netanya, Israel, from July 12-21, 2018, where he scored eleven goals in seven games, adding two assists.