Mr Sakaja received the instruments of power from outgoing governor Anne Kananu.
President-elect William Ruto graced Governor Sakaja's swearing-in ceremony at the Kenyatta International Convention Centre (KICC).
Governor Sakaja was elected on a UDA ticket after defeating his closest challenger Polycarp Igathe of Jubilee Party.
The incoming governors have promised to hit the ground running and deliver on promises they made during campaigns.
In Siaya, lawyer James Orengo was sworn in as the new governor.
Mr Orengo is taking over from outgoing governor Cornel Rasanga who has served for two terms. According to the Constitution, a governor can only serve for a maximum of two five-year terms.
There was song and dance during the swearing-in ceremonies.
In Nyeri, Governor Mutahi Kahiga was sworn in for a second term in a ceremony attended by outgoing Mathira MP Rigathi Gachagua who is now the deputy president-elect.
Justice James Wakiaga administered the oath of office to Governor Mutahi Kahiga in a ceremony held in Nyeri town.
In Vihiga, Governor Wilber Ottichilo was sworn in for his second and final term.
In Kirinyaga, Governor Anne Waiguru was sworn in to start her second terms in office in a ceremony presided over by Justice Hedwig Ong'udi