As well as a reduction in the number of councils, councils are being given more powers and responsibilities.
The changes, which represent the biggest shake up in local government in Northern Ireland for over 40 years, will modernise it and provide more effective and efficient services to ratepayers.
Environment Minister Mark H Durkan said: “April marks a new era for local government here. It represents real opportunities for change and improvement.
“This is about strong local government. It is about changing the way our councils are structured and the functions they provide. It is about giving councils more powers and flexibility to shape their areas and communities. It is about improving services to benefit those that use them.”
Acknowledging the roles that people have played in the programme, Mr Durkan added: “A remarkable amount of work has been done by all involved to get us to this point. I thank each and every person for playing their part.
“Change is always challenging and these are big changes. For many working on this programme, the changes directly affect them, making it an even more testing environment.
“I’d also like to thank the councillors and staff of the outgoing 26 councils as it is on the strength of their work, often during difficult and dangerous times, that the new councils will go forward.”
Mr Durkan added: “The real driver for this transformation will be the leadership, vision and openness to partnership working demonstrated by elected members and council staff.
“I wish all these people well and encourage them to continue to show their commitment to the journey ahead, ensuring that local government reform is the success that it has the chance to be.”