King replaces Jonathan Hill, who was in charge of financial services but resigned following Britain's shock decision to leave the EU in a referendum.
King's role will involve addressing the threat posed by returning foreign terrorist fighters; preventing and fighting radicalisation; improving information exchange and strengthening the European counter terrorism centre.
His nomination was approved last week by Parliament's civil liberties, justice and home affairs committee. King was questioned by MEPs on the committee in Strasbourg.
Committee Chair Claude Moraes described King, currently the UK's ambassador in Paris, as "highly competent", with a "sound knowledge" of his portfolio.
His job, said Moraes, will be to ensure that existing EU legislation in the field of security is implemented by national governments and to "work closely" with Parliament to ensure this is achieved.
Moraes said efforts were also needed to combat radicalisation, while fundamental rights will be "central to an effective security strategy."
The long-serving MEP said he was reassured to hear that King would carry out a "comprehensive" review of all existing EU security measures.
He said, "This will enable us to assess what measures work well and where legislation is not effectively implemented by member states."
King, expected to be the UK's last European Commissioner, will take office after he is formally appointed by ministers in the Council.
Greens/EFA group justice and home affairs spokesperson Jan Philipp Albrecht urged King to make security and the fight against terrorism "a priority."
Albrecht said, "The horrible attacks (in Paris and Brussels) exemplified that a European cooperation in security policies is overdue. King must use this opportunity to present sustainable solutions, instead of yet another placebo proposal.
"We need security policies based on reasonable suspicion instead of mass surveillance. King must take this as his starting point and provide solutions."
However, Ukip MEP Gerard Batten asked, "Why on Earth is Britain nominating a Commissioner at all, given that the last one had to resign as his position was untenable?"
King will be under the supervision of Commission First Vice-President Frans Timmermans until Brexit.