With all the precincts reporting, Buchanan had 58 percent of the vote compared to Phillips' 42 percent.
Buchanan said she is gratified by her victory for a third term in the Assembly.
"I'm really excited. I'm honored to be able to represent the people in the new 16th Assembly district," Buchanan said.
Buchanan said she looks forward to representing Dublin, Pleasanton, Lamorinda and other new communities in the new district.
She added she will focus on school funding during the next year in the state Legislature.
Phillips said he was grateful for the experience and all the people he's met even though he didn't come out victorious.
"I feel so enriched by the process," said Phillips. "It's been a fantastic experience. It's a wonderful experience. Everybody should do this. I am a better citizen and a better voter because of it."
The new Assembly district covers Walnut Creek, Lamorinda, Danville, San Ramon, Livermore, Pleasanton and Dublin.
Buchanan served 18 years on the San Ramon Valley Unified School District board of education. She is currently chairwoman of the Assembly Education Committee.
In November 2008, Buchanan received 52 percent of the vote. In November 2010, she garnered 53 percent. Both of those races were against former San Ramon Mayor H. Abram Wilson.
Phillips is a union carpenter who serves on the Bay Area Air Quality Management District board of directors.