He was born in Nyaribari location on 6 Feb 1932 to the then powerful colonial chief of the location Musa Nyandusi. Nyachae's earlier exposure to western education was facilitated by his own father, himself a mission educated graduate who used his strong influence and acquittance to the colonial administrators to be appointed chief. Born into a large polygamous family, Nyachae was fortunate that his father developed a particular liking for him, more than his elder brother James Oiruria, who apparently was less talkative and outgoing than Simon Nyachae. In 1941 his father put him in Nyanchwa Seventh-day Adventist School and laterin 1947 he joined Kereri intermediate school. However, two years later in 1949, he joined Kisii Government African School but in 1953 just a year before he sat for the then Ordinary Level School Certificate, he withdrew from the school and was employed at his father's chief camp as a district clerk in 1954.
Thus Simon Nyachae's long career in the civil service began at this point. However, in 1957 Nyachae's father realised that his son would be better off with higher education and that is why he arranged for his admission to study Public administration in London. Upon his return to Kenya in 1960 Nyachae was posted as a District Officer in Kangundo Division and later returning to Churchill College, Cambridge for a diploma course in public administration. He became a District commissioner by December 1963. Upon his return to Kenya in 1964 he went back to the Provincial administration and from this pointed through his aggressiveness, he steadily rose in the ladder within the Provincial Administration serving as Provincial Commissioner (between 1965–1979) and later chief secretary in the Civil Service, under the Kenyatta and Moi governments.
On his retirement from the civil service, Nyachae was easily one of the most prominent personalities from Kisii. When he was elected to parliament 1992 in Nyaribari Chache Constituency this was instrumental in his entry into the Moi government as a powerful cabinet minister first for Agriculture, then 1998 for Finance.
1999 he fell out with Moi and resigned from the government after having been moved to the less influential ministry of Industry. He also left Kenya African National Union (KANU) to join the opposition FORD People, by then only a small party with some roots in Central Kenya and three deputies in parliament.
Nyachae's plans to run for presidency did not find wide support as the main opposition groups cooperated with Mwai Kibaki's Democratic Party to form the National Alliance of Kenya which then teamed up with Raila Odinga's LDP to form the Rainbow Coalition.
During the 2002 General Elections he did not succeed in his presidential bid but was able to enter parliament with a 14 member strong FORD-People faction after collecting all constituencies in Kisii. Ford-People was the only sizeable opposition party besides KANU in the 2002 parliament.
Minister under Kibaki
When president Kibaki's NARC-Coalition started to crumble the support of FORD-People became most welcome and in 2004, when president Mwai Kibaki was facing strong opposition from his cabinet he recalled Nyachae to government as Minister for Energy and later for Roads.
On 31 May 2006 Nyachae declared intentions of retiring from politics due to his age.
Nevertheless he ran again for the 2007 elections. This time however, the opposition wave swept his Kisii stronghold and Nyachae was not elected again as it happened to the majority of Kibaki's government ministers and nearly all Ford-People candidates.
He runs a chain of business ranging from agriculture to industries based in Narok and Nairobi. He's one of the prominent businessman in Kisii town.
- The Standard, 4 April 2007: Gusii King of politics has had an undeterred political life