Former Army General and Democratic Party leader Fonseka, who was elected to the Sri Lankan Parliament earlier this month, took the oath of office before President Maithripala Sirisena at the Presidential Secretariat here.
The 65-year-old was made the Cabinet Minister for Regional Development.
The United National Party of Prime Minister Ranil Wickramasinghe named Fonseka to Parliament earlier this month as a national list member for a seat vacated by the death of Land Minister M K A D S Gunawardena.
Today's developments come over a year after the change in government in Sri Lanka that was earlier ruled by former strongman Mahinda Rajapkasa, with whom Fonseka had a fall-out.
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The former general had a fallout with Rajapaksa, under whom he had served, following which he was charged in a number of cases, ranging from corruption to engaging in politics in uniform.
He was also sentenced to 30 months in jail after having mounted a failed bid to unseat Rajapaksa in his January 2010 re-election.
Fonseka, who was nearly assassinated by a LTTE suicide bomber in 2006, was pardoned by Sirisena days after he assumed office as the new president last year. All ranks and medals denied to Fonseka by the Rajapaksa regime along with his pension were restored.