Mironov was the only candidature for the post in the party - which describes itself as a leftist force, but supports the Kremlin - receiving almost a unanimous backing by 235-2. The reelection was needed after the party adopted a new charter it said was designed to improve performance.
Speaking at the party congress, Mironov pledged to work for the good of the country as leader of A Just Russia: Rodina/Pensioners/Life.
Founded in October 2006 after the merger of three parties, the bloc suffered internal conflicts as many regional branches of the Party of Pensioners and the left-leaning Party of Life contested the leadership of Mironov's Rodina, the smallest of the three. Many members have since left the party.
The new charter consolidates the power of regional branches in the hands of chairmen, according to one of the delegates. Regional branches were earlier run by three leaders with no clearly divided responsibilities.
Mironov said at the congress: "Our party advocates ideological diversity and a multi-party system." He also expressed hope that outgoing President Vladimir Putin's decision to head the ruling United Russia party would not lead authorities across the country to clamp down on "initiatives not originating from United Russia."
Putin will take up the post of premier after stepping down next month. He will also be chairman of the country's largest party with a two third majority in parliament.
