Arizona obtained Pennington and minor league infielder Yordy Cabrera from Oakland for Young and cash, then sent Cabrera to the Miami Marlins for Bell and cash.
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 offseason but lost his closer's job in July. He finished with 19 saves in 27 chances and a 5.09 ERA in 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 the team experience at shortstop and second base.
Towers, who was 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.
Young, Arizona's center 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 center and be the Diamondbacks' leadoff hitter. But Towers said the job is not 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 talked to Billy (Beane) and got to talk to BoMel. I'm excited to help. It's a new opportunity, a fresh start for me personally.''
Pennington, 28, played in 125 games for the A's. He had 93 appearances at shortstop but was shifted to second base in last August. A switch hitter, Pennington hit .215 last season, including .168 against left-handers, and had 28 RBI.
''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.''
He said he believes Pennington can be an everyday shortstop plus fill in behind Aaron Hill at second. The team also has Willie Bloomquist and John McDonald at short.
Towers said that Bell may have been uncomfortable with his high-profile signing in the Miami makeover.
''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 offseason.
The Arizona general manager believes 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. 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 manage,might try to move center 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 favorite of mine.''
Oakland already acquired shortstop Stephen Drew from the Diamondbacks this season and he helped them down the stretch. The A's overtook Texas on the final day of the regular season to win the AL West 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 offseason, 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 after. A quadriceps injury in early September sent Young to the bench in favor of Eaton.