In an interview, Johanns rejected the notion that he was leaving because of heightened partisanship in the U.S. Senate.
"I've just always kind of had the attitude that you find ways to work together," Johanns said. "I know what the polling shows with Congress' approval ratings, and the anger and frustration that we all feel. But having said that, the people I worked with in the Senate were great. It wasn't frustration. This was just a decision that it's time."
Johanns, 62, joined the U.S. Senate in 2009 and did not appear to face any re-election threat. He served as agriculture secretary under Bush and was Nebraska's governor from 1999 to 2005.
The announcement came as a surprise to many GOP insiders. Several Republican office-holders praised Johanns for his collegiality and thoughtfulness in a deeply divided Washington. Johanns was a member of the "Gang of Eight" that tried to negotiate a federal deficit-reduction deal in 2011.
"I am personally grateful for Mike's leadership in the Senate and all he has done in helping to smooth my transition," said U.S. Sen. Deb Fischer, a Republican who was sworn in for her first term in January. "His rare mix of strong leadership and warm collegiality has earned him the respect and admiration of his colleagues on both sides of the aisle. While I am fortunate to have two more years to serve alongside him, I am sad to see Mike leave the Senate."
Johanns' decision to retire opens a new 2014 race in Nebraska, a solidly red state where Republicans hold all the congressional seats.
His departure could leave an opening for Republican Gov. Dave Heineman, who is leaving office in 2014 because of term limits. Heineman was courted by national party officials after Democratic U.S. Sen. Ben Nelson announced in 2011 that he was retiring. But Heineman declined to enter the race at the time, saying he wanted to focus on his work as governor.
In a statement, Heineman called Johanns "a remarkably outstanding public official."
"We are forever thankful for his commitment to serving our citizens, in particular, our farmers and ranchers, and our veterans," the governor said.
Although he began his political career as a Democrat, Johanns has made a name for himself as a staunch conservative since his election to the Senate in 2008.
In his first year in the Senate, Johanns voted with the GOP 94 percent of the time, including opposing the confirmation of Supreme Court Justice Sonia Sotomayor. He opposed the 2010 health care reform law championed by President Barack Obama, calling it bad policy even after the U.S. Supreme Court in 2012 upheld it as constitutional.
But unlike many Republicans, including Fischer, Johanns publicly backed former U.S. Sen. Chuck Hagel for secretary of defense.
In 2012, when it was revealed that the Environmental Protection Agency was using aerial surveillance to check farms in the Midwest for violations of federal clean water regulations, Johanns introduced an amendment to a multifaceted farm bill to stop the flights, but it fell four votes short of the 60 needed.
He began his foray into politics nearly three decades ago, when he was elected as a Democrat to the Lancaster County Board of Supervisors in southeast Nebraska. He switched parties in 1988, winning election as a Republican to the Lincoln City Council and, three years later, the city's mayor.
His political status — and conservative leanings — have only grown since. He was elected Nebraska's governor in 1998, and during his second term accepted Bush's appointment to U.S. secretary of agriculture. He stepped down in 2007 to launch his U.S. Senate bid.
During his tenure in the Senate, Johanns has eschewed earmarks, legislative amendments that allow members of Congress to add money for local projects to national bills in a process that is often obscured from public scrutiny.
Johanns faced a health scare during his first year as a senator, undergoing surgery in March 2009 after doctors found a spot on his left lung. No cancer was found, but doctors removed the lower left lobe of his lung as a precaution. Johanns was a smoker, but quit more than 20 years ago.
"Mike Johanns epitomized principled leadership during his near three decades of public service, always putting the needs of Nebraskans first," said U.S. Sen. Jerry Moran of Kansas, the chairman of the National Republican Senatorial Committee. "I thank him for his service and wish (him) and his family the very best in their next chapter. The Cornhusker state will look to continue its strong conservative tradition by electing an equally dedicated Republican leader in 2014."