Santos, who turns 20 on Christmas day, struck out 71 in 62 1/3 innings this past season, his first as a professional. He split time between the Arizona League and Class A short season Everett of the Northwest League.
• Hot Stove Tracker
The Mariners originally signed De Los Santos, who is from the Dominican Republic, for $15,000.
"Enyel has a big frame [6-foot-3] and a power arm and was one of the better guys in the Northwest League last season," Padres general manager A.J. Preller said on Thursday before leaving Florida, the site of the General Managers Meetings. "He has a chance to be a quality starter down the road."
Preller cited new director of pro scouting, Pete DeYoung, for his input on De Los Santos.
Ward, a 12th-round pick in the 2014 Draft out of the University of Georgia, has a career slash line of .270/.356/421. He split time in 2015 between the Mariners' Class A and Class A Advanced affiliates. He's a left-handed hitter.
"He's a middle infielder who has put up some numbers and gets on base," Preller said.
Preller cited Josh Emmerick, the team's West Coast scouting supervisor, for his work watching Ward.
The Padres recently exercised the $7.5 million option for Benoit, a precursor, as it turned out, to Thursday's deal.
Benoit, 38, had a 1.96 ERA in 120 games for the Padres over the last two seasons, mostly as the setup man for Huston Street in 2014 and Craig Kimbrel in '15.
Preller said the team will look to incumbents Kevin Quackenbush, Nick Vincent, Jon Edwards and possibly Brandon Maurer to help fortify the late innings -- that is if the team doesn't move Maurer into the starting-rotation mix.
Preller said outside candidates are a possibility as well.
"Year in and year out, you're always trying to fortify and add to your bullpen. It's always an area we're asking our pro scouting group about," Preller said.
Does the trade of Benoit mean the Padres are more likely to hang onto Kimbrel? Preller wouldn't say one way or the other.
Preller said Benoit was attractive to teams looking for bullpen upgrades without having to spend big in free agency. That his contract ($7.5 million) isn't exorbitant made him easier to move.
Preller said he had talks with 15 or so teams regarding Benoit.