He died of natural causes, according to his wife, Ione.
Monagan ended a 10-year dry spell for the state GOP when he rose to the speakership, succeeding Jesse M. Unruh, the formidable Democrat who had ruled the Assembly for a decade. Twenty-five years would pass before another Republican held the powerful post.
Elected to the Legislature in 1960, Monagan was a leader of a group of freshman Republican moderates known as the Young Turks, who weren't afraid to defy Unruh or the establishment in their own party. At the same time, he was known for his pragmatism and bipartisan approach that helped the Legislature run smoothly during an era of massive social change.
"He was an effective leader of the Republicans and effective as the speaker because the people on the other side of the aisle respected and trusted him as well," former Gov. Pete Wilson, who was a junior Republican assemblyman when Monagan was speaker, told The Times on Wednesday.
Steve Merksamer, who left college early to work in Monagan's legislative office and served as former Gov. George Deukmejian's chief of staff, called Monagan "the most consequential Republican speaker" of the last 50 years.
"There were people in the Legislature who were not smart, and there were people who were even corrupt and . . . excessively partisan," Merksamer told The Times on Wednesday, "but there was always a group of pragmatic centrists who made the place work. Bob exemplified that."
Monagan was born in Ogden, Utah, on July 5, 1920, and came to California when he was about 8 years old. The son of an electrician, he attended Vallejo High School, where he was an athlete and student body president, a role he also filled as a student at what is now the University of the Pacific in Stockton.
He entered the U.S. Coast Guard Academy after college and was a reserve officer in the Aleutian Islands during World War II. After the war he settled southwest of Stockton in Tracy, where he established an insurance and real estate business and won a seat on the City Council.
He was elected to the Assembly with a solid Democratic majority. Bill Boyarsky, a former Times political writer and columnist, wrote in a 2008 biography of Unruh, "Big Daddy," that Monagan's election was "among the first signs that California was shifting from the Franklin D. Roosevelt legacy of the Depression and World War II to something more conservative, a trend that would help elect Ronald Reagan governor in 1966."
Along with three other newly elected Republicans -- John Veneman, William Bagley and Houston Flournoy -- Monagan took on the Republican old guard, which they viewed as being too compliant with Unruh's agenda. They also took on Unruh, mobilizing other Republicans to oppose Gov. Pat Brown's proposed budget in 1962.
"We were all young and aggressive, and we were ready to roll," Monagan told Boyarsky, who reported that Unruh was so angry about the Young Turks' defiance that he blocked Monagan's bid to become minority leader.
Monagan eventually won the post and served as minority leader from 1965 to 1969. He also won Unruh's respect and campaigned with him to turn the Legislature into a full-time body.
Monagan also led efforts to pass the California Environmental Quality Act of 1970, which followed on the heels of federal efforts to require developers to document the environmental impact of their projects. "This was a keystone for the last 38 years" in environmental protection, Bagley told The Times on Wednesday. Monagan, in a show of bipartisanship that was characteristic of his leadership, invited Assemblyman Jack Knox, a Democrat from Richmond, to co-sponsor the bill.
"You wouldn't see that today," Bagley said, alluding to partisan bickering that has stymied action, such as timely passage of a state budget. Referring to the young Republicans who rallied around Monagan, he said: "We were problem-solving, moderate, progressive Republicans who believed in actually trying to govern the state," who reached across party lines to make state government accountable and effective.
Monagan served as speaker for two years, until January 1971. The next Republican to fill the job -- in 1995 -- was Doris Allen of Orange County, whose tenure was extremely brief.
Monagan left the Legislature in 1973 to accept a position as an assistant Transportation secretary in the Nixon administration. When he returned to California, he headed a number of special interest groups. He also served on a number of boards and commissions and was a longtime regent at the University of the Pacific.
In addition to his wife of 63 years, he is survived by two children, Michael and Marilee, both of Sacramento; two grandchildren; and four great-grandchildren.
A memorial service will be held at 2 p.m. Monday at East Lawn Mortuary, 5757 Greenback Lane, Sacramento.