Mr Sondhi declared his intention at an anti-government rally on Friday night but stopped short of calling on PAD members to join his campaign against the general election.
"There is no reason to vote in this election when we admit the political failure and the failed government," he told supporters of the Network of Students and People for the Reform of Thailand near Government House.
Mr Sondhi led the protests, which included the week-long seizure of Suvarnabhumi Airport, which helped bring down the government of Somchai Wongsawat, a brother-in-law of former premier Thaksin Shinawatra, in 2008.
The former yellow-shirt leader has kept a low profile since Suthep Thaugsuban began leading the People's Democratic Reform Committee (PDRC) campaign to oust the Yingluck Shinawatra government in late October.
The two, however, share a common position on seeing reform undertaken before a new election is held.
Mr Sondhi also urged the army chief, Gen Prayuth Chan-ocha, to make clear whether he intended to side with protesters or former premier Thaksin Shinawatra.
Mr Suthep has urged voters to sanction the election and said he would lead other protesters onto the streets on Sunday for a "picnic".
The former Democrat Party strategist and deputy premier insisted his followers would not impede voters from casting their ballots.
The cost to taxpayers of staging Sunday's election, which will not produce enough MPs to begin a House sitting, is 3.8 billion baht. Subsequent byelections and related activities to rectify the problem could take months and raise the cost further.