The president, in his condolence message, said he was deeply saddened by the news and prayed with Mary's family, relatives and friends.
"May our Lord bless and
comfort all of you during this time of grief," he said, a message that was also expressed by Deputy President William Ruto and ODM leader Raila Odinga.
The DP was among dozens of Kenyans who sent messages of condolences to the former Harambee Stars captain and his family following their loss.
Oliech's mother, famed for her fish servings at Mama Oliech restaurant in Nairobi, has been unwell for a while, under the care of Dennis.
Ruto mourned her as woman who nurtured a great son who ended up becoming
a leading Kenyan footballer who opened the
gates of professional football to budding players.
"This proved that focus,
commitment, diligence, effort are ingredients of success," the DP said.
For his part, Raila said he stands with Dennis, the entire
Oliech family and the soccer fraternity.
"We stand with the family in prayer at this
time of grief. May her soul forever rest in eternal peace," Raila said.
Nairobi Governor Mike Sonko, who recently appointed Dennis alongside Musa Otieno as members of the county Sports Committee, also sent his condolences.
He said: May the soul of your mother be at peace with our Heavenly Father.
May God give you the peace that you seek."
Embakasi East MP Babu Owino mourned Mama Oliech as a kind and generous woman who gave them a second home at her restaurant.
"She was known to most of us who found a second home at her restaurant as
Mama Oliech. We
will miss her dearly, may her soul rest in peace."
Starehe MP Charles Njagua said: "My thoughts and prayers are with you. May the Lord comfort you in this trying moment."
Kenyans, through Twitter posts, called upon the government to make medical care easily accessible and reasonably priced too in honour of Dennis' mother.
spent a lot of his hard money on his mothers' cancer treatment because
Kenya has failed to provide a basic right - healthcare. May she RIP," Nyokabi Kimani, a twitter user, said.