She met Chacko along with supporters and took primary membership of the party.
Lamba tweeted that she has taken receipt of Rs 5 and officially became a Congress member.
Lamba had met interim party President Sonia gandhi on Thursday.
Lamba quit the AAP on September 6 and had announced that she would join the Congress in the presence of Sonia Gandhi at her residence.
A former Delhi University President, Lamba had served in the Congress for 20 years before she quit the grand old party and joined the AAP and won the 2015 Assembly elections from Chandni Chowk.
On September 19, Lamba was disqualified from the Delhi Assembly by Speaker Ram Niwas Goel on the grounds of defection.
She was scheduled to join the party on Friday but later she later said that the joining has shifted to Saturday.