Pollock is on the injured list with a hamstring strain and prior to that had struggled after signing a one-year, $7 million free agent contract with Seattle in the offseason. The 35-year-old, who joins his third National League West team after spending the first 10 years of his career in Arizona and Los Angeles, is hitting .173/.225/.323 with five home runs in 138 plate appearances and lost his starting job in mid-May.
Known for punishing left-handed pitchers, Pollock has struggled against lefties this season, with an OPS of .378 against them and .762 against right-handers.
"As we looked for guys who could help us right now as we're a little banged up on the position player side, you're always going to kind of go back to your board of guys you've had interest in in the past, had interest in in the offseason," Giants president of baseball operations Farhan Zaidi said of acquiring Pollock. "Obviously he hasn't had a great season in Seattle but we kind of look at sort of some of the underlying measures -- plate discipline, of how he's moving around the field, his sprint speeds are good. We just view him as a really good change-of-scenery candidate."
Mathias, 28, joins his fourth organization this season. Texas traded him in spring training to Pittsburgh, which designated him for assignment and lost him to Seattle on a waiver claim in early July. In 47 games at Triple-A between Pittsburgh and Seattle, Mathias was hitting .299/.405/.427 with four home runs and 20 RBIs.
The Giants have been hunting for infield and outfield help on the trade market, though the imminent return of second baseman Thairo Estrada should strengthen them as they are in possession of the NL's top wild-card spot.
They did, however, lose outfielder Mike Yastrzemski to the injured list Monday after he aggravated a left hamstring issue that had prompted two earlier IL stints this season.
The Associated Press contributed to this story.