Soto, one of the first Latino women elected to statewide office from the Inland Empire, died at Woods Health Services in La Verne of complications from a stroke suffered in December.
She retired from public office last year after months of failing health.
In a statement Thursday, Gov. Arnold Schwarzenegger called Soto "a devoted public servant who committed her life to improving the lives of others in her community."
Los Angeles Mayor Antonio Villaraigosa called her "a passionate leader and a principled voice."
She followed her late husband, Phil, a pioneer in California Latino politics who served as an assemblyman in the mid-1960s.
Nell Soto served on the Pomona City Council from 1987 to 1998. But in 1993, she took the step that would characterize her career when she was appointed to the Air Quality Management District board.
Few Latino politicians had made a career of championing environmental protection. But Soto had watched Pomona Valley air pollution grow increasingly severe.
"She always reminded me, the first real environmentalists were Latinos because they were exposed . . . to all the chemicals" while working in the fields, said her son Tom.
In 2004, as a Democratic state senator, she formed a task force to help water districts in Fontana, Rialto and Colton address perchlorate contamination of groundwater.
"She was able to get Sen. [Dianne] Feinstein to deliver $20 million to [the] Inland Empire to buy replacement water" and clean up the pollution, her son said. "That was probably one of her last great efforts."
For many years, her political career was spent in the shadow of her husband.
He died in 1997. The next year, at 72, she won a seat in the state Assembly. Before the middle of her first term, Rep. George Brown died. State Sen. Joe Baca ran to replace him. She, in turn, won a special election to replace Baca.
Although her son Tom is a gay Democratic activist, she earned the gay community's ire in 1999 when, under attack from conservative and pro-family groups, she switched her vote on a proposal that would have protected gay students from harassment. The measure failed in the state Assembly by one vote.
Soto was born in Pomona on June 16, 1926, to a family that had lived in the area for six generations.
During World War II, she worked in a factory that made crates for bombs and another that made parachutes.
Also during the war, she met Phil Soto, a soldier on leave from Okinawa, Japan. They were married in 1946.
The couple ran television repair businesses in the city of Commerce and in an unincorporated area of Los Angeles County. They organized a campaign to incorporate the area, which is now the city of La Puente. Phil Soto became the town's first mayor.
In 1977, the family moved to Nell Soto's hometown of Pomona.
In addition to her son Tom, Soto is survived by three other sons, Michael, Patrick and Philip; a daughter, Anna; 11 grandchildren; and three great-grandchildren. Another son, Robert, died in 2004.
Services were pending.