McMahon was chosen by the party Wednesday night in the race to fill the post vacated by Republican Dan Donovan, who became a congressman in a special election earlier this month, said Tom Feeney, executive director for the party.
"He gives a tremendous opportunity to bring back this seat," Feeney said.
"He's well liked across Staten Island, he transcends party lines and he's been involved in Staten Island, both as an elected official and a community member, for five decades.
"It's a no brainer for us."
McMahon did not immediately respond to a request for comment.
Prior to working as a lawyer, McMahon represented Staten Island in Congress from 2009 until he lost re-election to Michael Grimm in 2012.
Before Congress, he served in the New York City Council and graduated from New York Law School, according to his online biography.
The DA seat was left empty after Donovan was sworn into Congress to replace Grimm, who resigned during his third term after he pleaded guilty to tax fraud in January.
Chief Assistant District Attorney Daniel Masters was sworn in as acting DA until the November race.
The Republican Party has not selected a candidate to run for the office yet and postponed a vote after Assemblyman Andrew Lanza told the committee he would not run, the Staten Island Advance reported.