Showalter took the American League prize for the third time after guiding Baltimore to its first division title in 17 years, and Williams snagged the NL honour following his first season as a big league skipper with Washington.
Showalter received 25 of 30 first-place votes and 132 points in balloting by the Baseball Writers' Association of America. He's established a unique pattern of winning once a decade following victories with the New York Yankees in 1994 and Texas in 2004.
"I won't be doing it 10 years from now," Showalter said on the MLB Network telecast.
Williams, who played under Showalter in Arizona from 1998-00, led the Nationals to an NL-best 96 wins. He got 18 first-place votes and 109 points, joining Houston's Hal Lanier (1986), San Francisco's Dusty Baker (1993) and Florida's Joe Girardi (2006) as the only men to win in their first seasons as a major league manager.
"This is an organizational award as far as I'm concerned," Williams said on a conference call. "It's a testament to how the organization has built itself."
Mike Scioscia of the Los Angeles Angels was second in the AL with four firsts and 61 points, and Kansas City's Ned Yost finished third with 41 points. Seattle's Lloyd McClendon followed with 29 points.
The 58-year-old Showalter piloted the Orioles to a 96-66 record and their first AL East crown since 1997 despite playing large chunks of the season without All-Stars Chris Davis, Manny Machado and Matt Wieters.
Voting took place before the playoffs, when Baltimore swept Detroit in the Division Series and then was swept by Kansas City in the AL Championship Series.
Until the ALCS, the Orioles had not lost four in a row since May and had not dropped consecutive home games since June 28-29.
Showalter became the first manager to win with three teams in one league. He is the third Orioles winner, following Frank Robinson in 1989 and Davey Johnson in 1997.
"I think you've got to keep in mind that (players) allow you to manage them in today's game. It's not like you get to do it because of your job title," Showalter said from his Texas home.
"It was a lot of fun to kind of get out of the way," Showalter added. "I had a great seat."
Pittsburgh's Clint Hurdle, who earned the NL honour last year, finished second to Williams with eight first-place votes and 80 points. Bruce Bochy of the World Series champion San Francisco Giants was third with three firsts and 30 points.
"For me, as a newcomer to the managerial fraternity, it is a privilege just to be considered amongst the best in our game. Clint and Bruce are certainly that," Williams said in a Nationals statement.
Miami's Mike Redmond also got a first-place vote and finished fifth, behind St. Louis' Mike Matheny.
A hard-nosed player and five-time All-Star over 17 seasons, Williams was coaching third base for Arizona when he was hired by Washington.
Now he is the franchise's fourth winner, joining Johnson (2012) and Montreal's Buck Rodgers (1987) and Felipe Alou (1994).
Williams credited his players, saying, "These guys made my transition easy."
The Nationals had hoped to contend for the Series title in 2013 under Johnson and came into this season with high expectations. Some predicted they would take the crown, which can often hinder a manager's chances of winning this award.
"What we accomplished this season would not have been possible without the right man at the helm," Nationals general manager Mike Rizzo said. "It was a pleasure to watch him grow throughout."
Williams stressed fundamentals from the start of spring training, and worked on creative defensive alignments. His biggest stamp might've come in late April when he benched young star Bryce Harper in the middle of a game for failing to run out a grounder.
The 48-year-old Williams kept the Nationals on track despite injuries to Harper, Doug Fister, Ryan Zimmerman and Wilson Ramos. Washington still won the NL East by a whopping 17 games, the biggest margin in the majors.
"As a staff, we tried to empower our players and allow them to have a voice and decide how they're going to go about doing it," Williams said. "So, these guys did it. I didn't do anything except try to guide that at certain times."
The Nationals lost to the Giants in four games in the NL Division Series.
AP Sports Writers Ronald Blum and Howie Rumberg, and AP Baseball Writer Ben Walker contributed to this report.Suggest a correction