Former First Lady Lucy Kibaki died at a London hospital on Tuesday morning after being unwell for about a month.
Lucy, who was the wife of former president Mwai Kibaki, died aged 82 at Bupa Cromwell Hospital.
The former First Lady fell ill in March and was admitted at the Nairobi Hospital's Intensive Care Unit.
Lucy had earlier been taken to Gertrude's Children's Hospital for emergency care. Her family said she was suffering from age-related complications.
She was flown out of the country over the weekend for further treatment.
In a condolence message on Tuesday, President Uhuru Kenyatta said she will be remembered for her immense contributions to the country's development.
"Especially notable were her Excellency's virtues of leadership and commitment to improving the well-being of ordinary Kenyans. Her focus on tackling HIV/Aids in particular will remain an inspiration to many," he said.
The former First Lady chaired the Organization of the 40 African First Ladies Against HIV/Aids.
She was also known for supporting disadvantaged and disabled people. Kibaki officiated the opening of a hospital named 'Mama Lucy Kibaki'.
In a message via Twitter, DP William Ruto expressed deep sorrow upon learning of Lucy's death.
Amani leader Musalia Mudavadi said in a statement that for 10 years, Lucy "exemplified and stood studiously for the place and voice of women in society".
"Some may have been uncomfortable with her iron lady status but she carried with her the motto that behind a successful man, there is always an unapologetic woman."
Mudavadi said Lucy stood by her husband through adversities and never let go of the maternal instinct to protect her family.
"She left that imprint on many a Kenyan woman," he said. "All Kenyans should join the Kibaki family in mourning the passage of a matriarch. May God rest her soul in eternal peace."
Via Twitter, National Assembly Majority leader Aden Duale prayed for strength for the family while Homa Bay woman representative Gladys Wanga prayed for comfort.
Lucy was rushed to hospital in 2014 after reportedly complaining of chest pains.
She maintained an "uncharacteristically low profile" and did not attend any public functions during part of her time as First Lady.
The last public function she attended was the ushering in of the new constitution at Uhuru Park, during which she took part in a lively and energetic jig.
Lucy was born in 1934 in Mukurweini to Reverend John Kagai and Rose Nyachomba.
She attended Alliance Girls High School then trained as a teacher. She later became the principal of a teacher training college in Kiambu.
Lucy got married to the former president in 1962. They had four children - Judy Wanjiku, Jimmy Kibaki, David Kagai and Tony Githinji.