Henderson, 35, was placed in interim charge following the departure of Grima due to family reasons, having joined the club as assistant coach in the off-season.
He said: “The role has come earlier than expected but I don’t think there’s ever a right time to become head coach, it’s more when the right opportunity is presented. It’s a fantastic opportunity for me here at the London Broncos.
“I believe the current squad has a lot of unfilled potential and I’m hoping my staff and I will see the best of the squad in the coming weeks and months.
“My job now is to improve the confidence and belief of the current players within the squad to deliver better performances as a team and to continue to strengthen the squad moving forwards.
“Part of my vision is to continue the development of London based players and build the future around our kids, I’ll also look to work closely with our general manager Jason Loubser to improve and strengthen our infrastructure within the club for the long term. We need to earn the right to be back in Super League and build the club accordingly.”
Henderson played for Sheffield, Salford, Castleford, Gateshead, Barrow and Balmain during his playing career.