Arizona obtained Pennington and minor league infielder Yordy Cabrera from Oakland for Young and cash, then sent Cabrera to Miami for Bell. The Marlins will pay $8 million of the remaining $21 million Bell is owed.
Earlier in the day, Arizona exercised a $6.5 million option on closer J.J. Putz.
The 35-year-old Bell has 151 saves the past four seasons. He signed a $27 million, three-year contract with Miami last off-season but lost his closer's job in July. He finished with 19 saves in 27 chances and a 5.09 ERA in 73 games.
After failing to contend in the first season of their new ballpark, the Marlins traded former NL batting champion Hanley Ramirez to the Los Angeles Dodgers in July and sent pitcher Anibal Sanchez and infielder Omar Infante to the Detroit Tigers.
Arizona general manager Kevin Towers said Bell provides a right-handed power arm to help set up Putz, and Pennington adds experience at shortstop and second base.
Towers, who was the general manager in San Diego when Bell was the setup man for Trevor Hoffman, believes the pitcher will benefit from a return to the NL West.
"I think he's excited to kind of be able to clean the slate," Towers said.
Bell's contract calls for a $9 million salary in each of the next two seasons and includes a $9 million club option for 2015. Miami will pay $1.5 million next year, $3.5 million in 2014 and the $3 million deferred signing bonus he is owed.
Young, Arizona's centre fielder for six seasons, was the odd man out in a crowded outfield with the team expecting young Adam Eaton, called up from Triple-A Reno late this season, to play centre and be the Diamondbacks' leadoff hitter. But Towers said the job isn't automatically Eaton's. Gerardo Parra and A.J. Pollock also will compete for the spot.
Young will be reunited with Bob Melvin, who was his manager in Arizona.
"It caught me off guard a little bit, I'm not going to lie," Young said. "It's all settled in a couple hours now. I got an opportunity to talk to Billy (Beane) and got to talk to (Melvin). I'm excited to help. It's a new opportunity, a fresh start for me personally."
The 28-year-old Pennington played in 125 games for the AL West champion A's this season. He had 93 appearances at shortstop but was shifted to second base in August. A switch-hitter, Pennington hit .215 this year — including .168 against left-handers — and had 28 RBIs.
"He had a down year. That happens. Guys have down years," Towers said. "Heath had a down year. That's why we were able to make these deals."
Towers said he believes Pennington can be an everyday shortstop, plus fill in behind Aaron Hill at second base. The team also has Willie Bloomquist and John McDonald at shortstop.
Towers said Bell might have been uncomfortable with his high-profile signing during the Miami makeover last winter.
"Maybe a lot of it had to do with the pressure, his first big contract," Towers said, adding that Bell "was really the first big signing" the Marlins had in the off-season.
The Arizona general manager thinks his bullpen — led by Putz, David Hernandez, Brad Ziegler and now Bell — "is as good a bullpen as there is if not in the National League but in baseball."
With considerable money invested in the bullpen, Towers said it's particularly important to have a strong group of relievers while Arizona's young starting rotation develops. He said Arizona still needs a late-inning left-handed reliever and perhaps a veteran starter.
This could be a sign Beane, the A's general manager, might try to move centre fielder Coco Crisp, who signed a $14 million, two-year contract in January that includes a $7.5 million club option for 2014 with a $1 million buyout. Beane, however, said he plans to keep Crisp, though he realized there would be speculation otherwise.
"I really like our outfield," Beane said. "Everybody knows how important this guy is to this team and also has the benefit of being a personal favourite of mine."
Oakland already acquired shortstop Stephen Drew from the Diamondbacks this season and he helped the Athletics down the stretch. The A's overtook Texas on the final day of the regular season to win the division and returned to the playoffs for the first time since 2006. They lost in five games to the World Series-bound Detroit Tigers.
Beane said the day after the season ended he planned to keep his young team intact as much as possible. Last off-season, Oakland traded three top pitchers: Trevor Cahill to Arizona, Gio Gonzalez to Washington and closer Andrew Bailey to Boston.
The 29-year-old Young, a popular player in the Arizona clubhouse, has had three 20-homer, 20-steal seasons. He injured his right shoulder crashing into a wall while making a catch early this season and struggled at the plate afterward. A quadriceps injury in early September sent Young to the bench in favour of Eaton.
AP Baseball Writer Janie McCauley and AP Sports Writer Ronald Blum contributed to this report.