"Catherine was a wonderful person and that really made her a fine public servant, both as state treasurer and lieutenant governor," Rendell told The Associated Press. "She cared as deeply about people and the problems they faced as much as any public official who served the commonwealth."
Knoll was diagnosed with neuroendocrine cancer in July 2008 and began radiation and chemotherapy treatments before publicly revealing her illness in August.
The Senate's presiding officer, she returned for the start of the fall Senate schedule in September, but showed signs of fatigue. On Sept. 22, she announced she would heed the advice of doctors, family members and colleagues and take time off, missing her first Senate session in six years.
Rendell said her death surprised her family, because she had shown progress on Wednesday, standing up for the first time while rehabilitating from a severely weakened condition.
"She went down fighting, like she did all her career and all her life," Rendell said.
In a statement, her family said she loved working for the people of Pennsylvania and was proud of the friendships she had forged throughout the state.
"She fought this illness with the same tenacity she brought to a lifetime of public service," her son Albert Baker Knoll said. A former schoolteacher and Democratic veteran, Knoll won two terms as state treasurer, in 1988 and 1992, while running unsuccessfully three other times. In 2002, she beat out eight other candidates in the Democratic primary for lieutenant governor, and went on to win the office as Rendell's running mate.
Rendell said Knoll's broad constituency and name recognition had helped him appeal to her home turf of western Pennsylvania. She was fond of saying her age had brought wisdom, and that she needed no on-the-job training.
"I happen to think that Pennsylvania is like a 10-speed bicycle," Knoll said on the day she was sworn in as the state's 45th lieutenant governor in 2003. "We have gears we haven't even tried yet."
As lieutenant governor, she held the gavel in the state Senate and chaired the state Board of Pardons, her primary tasks.
She was so committed to presiding over the Senate that, even cancer-stricken, she told Senate Minority Leader Robert J. Mellow, D-Lackawanna, in September that she felt badly about putting down the gavel, in case she was needed to cast a tie-breaking vote for the party.
Mellow described a determined woman who was passionate about being an elected official - she would go anywhere, any time to represent the state, he said - and serving the Catholic church. "If there is a Heaven and Hell, then when she passed away she went straight to Heaven," Mellow said.
Under the state Constitution, Knoll will be replaced by the Senate President Pro Tempore, Joseph B. Scarnati III, R-Jefferson, who plans to retain his Senate seat.
In a statement, Scarnati said Knoll "embodied the type of character expected of true, effective public servants."
U.S. Sen. Bob Casey called her life "one of service and sacrifice, courage and commitment to the common good."
Knoll's tenure as treasurer had been damaged by allegations that a former campaign aide used his position to benefit from the sale of state bonds. Knoll was never implicated, but it came up repeatedly in later campaigns.
Knoll was born in Sept. 3, 1930. Her father, Nicholas Baker, was the mayor of McKees Rocks, a Pittsburgh suburb. She originally tried nursing school, but disliked it, and went to Duquesne University to study history and education.
She met her husband, Charles Knoll, while she was a student and married him just before graduating.
She worked for local Democratic candidates, became a member of the party's state committee and started working for PennDOT in the early 1970s.
In 1976, the party asked her to run for state treasurer. She lost, ran again in 1984 and lost in the primary by fewer than 15,000 votes.
She pledged to never run for office again, but changed her mind when her husband, a postmaster, died in 1987. All four of their children encouraged her to do so. Knoll, affectionately known as CBK, won handily and pledged to clean up a treasurer's office that she said was a mess. She said she was proud the agency provided $25 million in loans to small businesses through development centers at colleges and universities, as well as $100 million in low-rate first-home mortgages to single parents, first-time buyers and veterans. She also oversaw the startup of a college savings program for parents.
Aside from her long service, Knoll was known for raising eyebrows with an occasional gaffe.
She once agreed to be the guest of honor at a fundraiser for a high-ranking Republican and was criticized by Democrats for agreeing to co-sponsor a fundraiser for another GOP state representative. Knoll ultimately canceled both plans.
In 2005, Knoll apologized to the family of a Marine killed in Iraq for showing up uninvited for his funeral and giving out a business card. Family members said she made a remark about "our government" being against the war. It was, Rendell said, a mistake of the heart by someone who wanted to be called if she could alleviate suffering or ease burdens.
Also that year, she introduced an Amazon parrot to the Senate floor. The parrot, from the national Aviary in Pittsburgh, warbled "How much is that doggie in the window?" for stunned lawmakers, some of who were angered by what they viewed as a breach of protocol.
No funeral arrangements were made public Wednesday. Knoll is survived by three sons, Albert, Kim Eric, and Charles, and a daughter, Mina.