The Congress last came to power in the state in 2004 as part of a coalition government with the Janata Dal (Secular) - but now has a clear majority in the Assembly after its recent win in state Assembly elections.
Mr Siddaramaiah was chosen on May 10 as the Congress Legislature Party leader in a hotly-contested race, edging out party veteran and Union Labour Minister M Mallikarjuna Kharge.
Though Mr Siddaramaiah was the front-runner for the coveted post, his selection came only after a meeting between AICC observers, headed by Defence Minister AK Antony, and party legislators.
Mr Siddaramaiah had joined Congress from JD (S) six years ago and was a Deputy Chief Minister under N Dharam Singh.
A lawyer-turned politician, Siddaramaiah had narrowly missed the Chief Minister's "gaddi" in 1996 after the incumbent H D Deve Gowda went on to become Prime Minister.
Siddaramaiah, who hails from Kuruba community, the third largest in the state, was then picked by J H Patel. Both under Mr Deve Gowda and Mr Patel, he served as Finance Minister.
Siddaramaiah and state Congress chief G Parameshwara are expected to meet party brass in Delhi later today to get a final nod for ministry formation.
Battered by a series of scams at the national stage, Congress put up an impressive show in the May 8 Assembly elections winning 121 seats, unseating the BJP.
Election has been postponed in Periyapatna Assembly constituency due to the demise of BJP candidate Sanmoggegowda.
The poll is scheduled on May 28 and counting will take place on May 31.