She graduated from Gresham High School and received a bachelor's degree in biology from Willamette University in 1968, and a master's in biology from the University of Colorado in 1972.
She was reelected in the 2008 Oregon legislative elections. In 1981, she earned a degree in nursing from Radford University. Following a career as a public health nurse, Monnes Anderson began serving on the Gresham-Barlow School District board in 1991.
In 1998, she ran for the Oregon House of Representatives representing District 22, but lost to incumbent Ron Sunseri by 250 votes. In 2000, Sunseri did not seek re-election, and Monnes Anderson defeated Republican nominee Ed Golobay for the seat in a close election. She was re-elected to the seat (renumbered to District 50) by a wide margin in 2002 over Ernest Hodgin.
In 2004, she sought election to the Oregon Senate in District 25. In this race, she again faced Ron Sunseri, who had defeated her in her first House race in 1998. However, this time, Monnes Anderson prevailed by 9 percentage points in an expensive race.
She defeated Republican Dave Kim in the general election. Monnes Anderson lives in Gresham, Oregon. She is a first cousin of cartoonist Matt Groening, the creator of The Simpsons.
Monnes Anderson's mother, Ellene Groening Monnes, is the sister of Groening's father Homer.
She was unopposed for her party's nomination in 2008.