Ms Margaret Nyang'ate Nyakang'o was on Wednesday sworn-in as the second Controller of Budget (CoB) under the current Constitution.
President Uhuru Kenyatta earlier appointed her as the successor of Agnes Odhiambo, whose tenure ended on August 27.
The President announcement the appointment in a Gazette notice dated December 4.
Chief Justice David Maraga, who presided over the ceremony at the Supreme Court building, challenged Ms Nyakang'o to uphold the rule of law and the Constitution.
Members of Parliament approved Ms Nyakang'o's nomination on Tuesday, following her vetting last week.
The Finance and National Planning Committee, in its report tabled in the National Assembly, said she was well-suited for the job.
Ms Nyakang'o's immediate tasks include investigating why 15 county governments have defaulted paying suppliers, a circumstance that has prompted the National Treasury to request Parliament to stop transfers.
She will be in charge of withdrawals from the public kitty and managing devolved units' expenditure.
Counties have been exceeding set limits on monthly spending for ward representatives' sitting allowances and foreign and domestic travel.
The 47 county governments have consistently missed their revenue targets since the advent of devolution of resources and services under the new Constitution in 2013.
Ms Odhiambo also raised the red flag on the ballooning public expenditure largely driven by a bloated workforce both in the national and county governments.
Under the CoB Act, any Kenyan citizen can serve as the Controller of Budget as long as he or she holds a degree in finance, accounting or economics, and meets the constitutional threshold for integrity.