Severino began his professional career in 2006, pitching for the DSL Nationals. He appeared in 13 games, eight of which were starts, going 2-0 with a 0.99 ERA in 45 1⁄3 innings, striking out 69 batters.
In 2007, he pitched for the DSL Nationals (3-0, 0.48 ERA in 3 games started) and GCL Nationals (1-0, 2.94 ERA in 13 games, five starts), going a combined 4-0 with a 2.06 ERA in 16 games (eight starts). He had 59 strikeouts in 52 1⁄3 innings.
He split the 2008 season between the Potomac Nationals (0-4, 3.96 ERA in 26 relief appearances) and Hagerstown Suns (4-2, 4.05 ERA in 15 relief appearances), going a combined 4-6 with a 4.00 ERA in 41 relief appearances.
He improved in 2009, as he split the season between Potomac (4-0, 2.54 ERA in 29 games) and Harrisburg Senators (6-0, 2.78 ERA in 15 games), going a combined 10-0 with a 2.62 ERA in 44 relief appearances.
He appeared on the major league roster with the Nationals for three days in 2010, but did not make a pitching appearance at that level.
Severino made his major league debut on September 6, 2011. He appeared as a reliever in 6 games in 2011, going 1-0 with a 3.86 ERA in 4 2⁄3 innings, striking out 7 and walking 1.
In 2012 Severino made 46 relief appearances the Nationals Triple-A Syracuse team, posting a record of 3-0 with a 2.81 ERA plus three saves. He was designated for assignment on August 6, 2012.
On November 16, 2012 the Kansas City Royals announced they had signed Severino to a minor-league contract for the 2013 season. No financial terms of the deal were announced. Severino was traded to the Pittsburgh Pirates on May 30, 2013, in exchange for cash considerations. He signed a minor league deal with the Atlanta Braves in December 2013.
On April 1, 2016, Severino signed with the Sultanes de Monterrey of the Mexican Baseball League. He became a free agent after the 2016 season.