The brother-in-law of exiled former premier Thaksin Shinawatra, who was ousted in a coup in 2006, Somchai faced a fierce protest campaign by opponents who loathed his more famous relative.
A respected bureaucrat and former judge before assuming power in September, Mr Somchai was on Tuesday barred from politics for five years by Thailand's constitutional court in a vote fraud case.
The bespectacled 61-year-old was thrust into the spotlight when he was chosen to replace the abrasive Samak Sundaravej, after Samak was removed from office for receiving payments for hosting a cooking show.
Despite months in the firing line, Somchai's nightly television addresses as the country unravelled last week were a study in calm - bowing solemnly to the camera before insisting that he was seeking a negotiated outcome.
But analyst Chris Baker said Mr Somchai's marriage to Thaksin's politically powerful sister Yaowapa meant he was always going to be a controversial choice for prime minister.
'It was obviously a defiant move to choose him in the first place and certainly with this (party dissolution) case pending,' Mr Baker told AFP.
'This was tempting this sort of conclusion.'
Mr Somchai only entered politics after his wife was barred by a court from running for office following the 2006 coup against Thaksin. Mr Somchai and his daughter both won seats in parliament in last December's elections.
'He was a senior bureaucrat who gets along with the politicians very well,' said political analyst Thitinan Pongsudhirak.
Mr Somchai was first appointed to a senior position in the bureaucracy under the Democrat Party, who are now in the opposition.
He held the highest-ranking positions at Thailand's justice and labour ministries, and for the past year has served as education minister and deputy party leader.
Any politician from the PPP would be immediately branded a proxy for Thaksin, Mr Thitinan said, nevermind Mr Somchai's family links.
On being forced out on Tuesday, Mr Somchai characteristically said he accepted the court's decision - and then headed off for lunch.
'My duty is over. I am now an ordinary citizen,' Mr Somchai told reporters in the northern city of Chiang Mai, from where he has been governing since an opposition blockade of Bangkok's airports began last week.
'But it is unexpected that the decision would come out this way. In the past I have done my best, not for myself but for our country,' he added. -- AFP