Rabbitt, who defeated Petaluma Mayor Pam Torliatt for the Second District seat in November, paid homage to his 88-year-old father, who moved to America from Ireland "with $40 in his pocket and a change of clothes."
He acknowledged the board of supervisors will make decisions some county residents might not like.
"But they will be thoughtful and vetted thoroughly," Rabbitt said.
He also mentioned the budget constraints the state and county currently face but said, "Things will brighten up."
"You have to know when the campaign is over and it's time to govern. It's time to govern," Rabbitt said.
McGuire, a Healdsburg city councilman, defeated Windsor City Councilwoman Debra Fudge in the June primary election for the Fourth District seat.
"This is a heck of a day," he said shortly after his wife administered the oath of office 31 days after they were married.
He said the challenging times the county faces "will bring us together."
"We have no other choice," he said. "We're in this together."
Also sworn in this morning were District Attorney Jill Ravitch; Sheriff Steve Freitas; Auditor, Controller, Tax Collector and Treasurer Rod Dole; Clerk, Recorder and Assessor Janice Atkinson; and County Counsel Bruce Goldstein.
The board of supervisors selected Supervisor Efren Carrillo as chair and Supervisor Shirlee Zane vice chair of the board. Both have been on the board for two years.
Supervisor Valerie Brown has been on the board since 2002.