In a limited cabinet expansion, Governor Nikhil Kumar administered oath of office and secrecy to 57-year-old Chennithala.
He is tipped to get the Home portfolio in the ministry, replacing his party colleague Thiruvanchur Radhakrishnan, who has been retained in the cabinet.
Chief Minister Chandy, his cabinet colleagues, senior party functionaries, leaders of the UDF partners and deputy leader of opposition in the assembly Kodiyeri Balakrishnan were present at the function.
Influential Nair outfit NSS had in the past had come down heavily on UDF alleging that the coalition leadership breached its poll-eve promise of giving Chennithala a "key position" in the cabinet, in which nominees from Christian and Muslim communities outnumber those from various Hindu sections.
The idea to rope in Chennithala, mooted by the party high command, was endorsed by the state unit yesterday after senior leader and Defence Minister A K Antony conveyed it at a leadership meeting attended by Chandy, Chennithala and others.