The club has signed 27 of the 41 players from last week's First-Year Player Draft, including seven of its top 10 picks and 25 of its first 30.
DeCarlo joined Mariners batting practice prior to the team's game against the Padres on Wednesday. He will soon join the team's Arizona Rookie League squad.
"It's awesome," DeCarlo said. "The ball doesn't carry, but it was an awesome time I had today. I always heard it was a beautiful ballpark, and it sure is.
"Ready to go to Arizona and get crackin'. ... I'm just worried about playing ball, and excited to be a Mariner."
DeCarlo signed for $1.3 million, which is above the assigned second-round value of $806,000.
Catcher Mike Zunino -- the Mariners' top pick and the third choice overall in the Draft -- has not signed, as his University of Florida team is playing in the College World Series. Also yet to agree are third-rounders Edwin Diaz and Tyler Pike, both pitchers.
Fourth-rounder Patrick Kivlehan and seventh-rounder Taylor Ard had already been announced. The new additions in the top 10 rounds are DeCarlo (second), Chris Taylor (fifth), Tim Lopes (sixth), Jamodrick McGruder (ninth) and Grady Wood (10th).
"We are excited to get these players in uniform and to see all the young talent we recently drafted in action," director of amateur scouting Tom McNamara said.
Here's the full list of players signed:
Right-handed pitchers: Wood (10th), Blake Hauser (13th), Dominic Leone (16th), Levi Dean (23rd), Matt Vedo (24th), Mark Bordonaro (25th), Aaron Brooks (26th), Matt Brazis (28th)
Left-handed pitchers: Nate Koneski (19th), Steven Ewing (20th), Scott DeCecco (21st), Blake Holovach (27th), Rusty Shellhorn (31st)
Catcher: Toby Demello (29th)
Infielders: DeCarlo (second), Kivlehan (fourth), Taylor (fifth), Lopes (sixth), Ard (eighth), McGruder (ninth), Kristian Brito (11th), Brock Hebert (14th), Gabrial Franca (22nd), Richard Palase (32nd)
Outfielders: Michael Faulkner (12th), Dario Pizzano (15th), Jabari Henry (18th).
Teams have until July 13 at 2 p.m. PT to sign players selected in this year's Draft.
Greg Johns is a reporter for MLB.com. Follow him on Twitter @GregJohnsMLB as well as his Mariners Musings blog. Josh Liebeskind is an associate reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.