Three of Russia's smaller political parties merged into A Just Russia party at a unification congress Saturday.
A total of 160 congress delegates out of 166 voted for Mironov's candidacy.
The tri-party alliance is being formed to fight for seats at regional and parliamentary elections next year.
Mironov, who leads the Party of Life and is the speaker of the Federation Council, earlier said the new party would have about 500,000 members, which would bring it close to Russia's traditional left-wing party, the Communist Party, and its 580,000 members.
"The time of small parties is over," he said.
He said the party could win the necessary 7% of the vote needed to enter the lower chamber of parliament, the State Duma, as it could take up a left of center niche where many voters' sympathies lie. He also suggested that it would be given access to airtime on the nationwide broadcasting channels.
Neither the Party of Life nor the Pensioners won enough votes in the 2003 election to take up seats in the State Duma. Rodina, which stormed into the Duma at the first attempt with just over 9% of the vote, has since been beset by problems.
Mironov said the new party would take up opposition to United Russia, the current "party of power," which holds a massive majority in the Duma.