During a news conference on Friday, Murphy said Helmy will be able to serve the residents of New Jersey right away.
"George understands on a fundamental level how a U.S. Senate office operates, and he will be able to run this office from moment one," Murphy said.
Though he will only serve for a few months, Helmy said he hopes his presence will have a lasting impact.
"If nothing else from my short time in office, I hope I begin to restore a small measure of faith and belief in the honor of public service," Helmy said.
Helmy served as Murphy's top aide from 2019 to 2023 and is now an executive at RWJ Barnabas Health, a large health care provider in the state. He was formerly an aide to Sen. Cory Booker of New Jersey.
His appointment means Murphy passed over Rep. Andy Kim, who is the Democratic nominee for the seat on the November ballot, facing off against Republican hotelier Curtis Bashaw. Kim was briefly locked in a tough primary with first lady Tammy Murphy before she dropped her bid. Murphy's choice denies Kim a boost in seniority if he were to win the seat in November.
Menendez wrote in a letter to Murphy last month that he's resigning on Aug. 20, just over a month after the jury's verdict. He said he planned to appeal.
Menendez, 70, was convicted of charges that he sold the power of his office to three New Jersey businessmen who sought a variety of favors. Prosecutors said Menendez used his influence to meddle in three different state and federal criminal investigations to protect his associates. They said he helped one bribe-paying friend get a multimillion-dollar deal with a Qatari investment fund and another keep a contract to provide religious certification for meat bound for Egypt.
Calls for Sen. Menendez to step down after conviction in corruption trial
He was also convicted of taking actions that benefited Egypt's government in exchange for bribes, including providing details on personnel at the U.S. embassy in Cairo, ghostwriting a letter to fellow senators regarding lifting a hold on military aid to Egypt. FBI agents found stacks of gold bars and $480,000 hidden in Menendez's house.
After his conviction, Menendez denied all of those allegations, saying "I have never been anything but a patriot of my country and for my country. I have never, ever been a foreign agent."
Numerous fellow Democrats had urged him to resign, including Senate Majority Leader Chuck Schumer. Murphy had urged the Senate to expel Menendez if he didn't quit. Only 15 senators have ever been expelled. Sen. William Blount, of Tennessee, was ousted in 1797 for treason. The other 14 were expelled in 1861 and 1862 for supporting Confederates during the Civil War.
Menendez's sentencing is scheduled for late October, shortly before the election.