Ms Sawyer took the oath of office at a ceremony presided over by Vice President Dr Juldeh Jalloh.
Ms Sawyer was elected in the March 7 general elections, after defeating about three other contenders.
She was elected with nearly 60 percent.
Her main rival, Sierra Leone People’s Party’s Raymond de’Souza George, has since been appointed cabinet Minister by President Julius Maada Bio.
Her swearing in comes amidst deafening complaints from supporters of her opposition All People’s Congress (APC) party over the protracted delay in the process.
Sawyer is the third woman to have held the position of Mayoress of Freetown.
The chartered accountant becomes the first female mayor since 1980 when the position was last held by Dr June Holst-Roness.
Yvonne Aki-Sawyer rose to fame during the deadly Ebola epidemic of 2014-2015, when she served as part of a crop of UK-funded experts flown into the country to help control the disease which ended up claiming nearly 4000 lives and killed about that same number.
For her role in the Ebola fight, she was honored by the Queen of England with the Order of the British Empire (OBE) insignia.
At home she also received a presidential medal for her role in fighting the epidemic. Sawyer is said to also hold an MSc in Politics of the World Economy from the London School of Economics, and a BSc Hons in Economics from Fourah Bay College, University of Sierra Leone.
The new Freetown mayoress is married with two children.
Her swearing in was greeted with euphoria especially among women’s rights and gender activists.
However, she faces the daunting task of leading one of Africa’s most disorganized capital cities, where service demand far outnumber the services provided.