Roy was earlier associated with TMC during its formation in 1998.
Besides Roy, Arun Das, another Congress Councillor and local party leader in Central Kolkata, also joined TMC.
Das had once been suspended from Congress in May, 2005 after the then External Affairs Minister Pranab Mukherjee was heckled by partymen over distribution of tickets for the Kolkata civic polls .
Flanked by the two councillors, TMC secretary-general Partha Chatterjee told a press conference "Two Congress leaders, Mala Roy and Arun Das have been inducted as Trinamool members".
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"Roy needs no introduction. She is the councillor for about two decades while Das has served as a councillor for 30 years", Chatterjee said.
Roy said "there is a lot of scope to work in Trinamool Congress."
Asked how she felt about leaving Congress where she was a bitter critic of Mamata Banerjee, Roy shot back saying "I was among those who formed Trinamool Congress. I was with TMC which was formed after a lot of struggle. I am happy to return to my old party".
Roy would seek re-election from ward 88 in the coming polls to the 144-ward strong KMC, Chatterjee said.
Das, who was once a close associate of an influential state Congress leader and former WBPCC chief Somen Mitra, represents ward 55 in central Kolkata.
On the quitting of the party by Roy and Das, PCC president Adhir Chowdhury said "probably they wanted to be with the party in power."
Chowdhury also questioned their timing to switch over to TMC just days before the election describing it a "sheer opportunism".