The report credited more effective and affordable treatments, new ways of delivering healthcare to the poor, as well as political commitment for the gains.
But it also underscored that much remained to be done.
"This trend is a positive one. Millions of lives have been saved," said Anthony Lake, UNICEF's executive director.
However, "most of these deaths can be prevented, using simple steps that many countries have already put in place. What we need is a greater sense of urgency."
All regions except Oceania and sub-Saharan Africa saw more than a 50 percent decline.
In eastern Asia, early childhood deaths have dropped by 74 percent and in Northern Africa the decline was 69 percent.
But child mortality rates were still elevated in some regions, with around 80 percent of under-five deaths in 2012 still taking place in sub-Saharan Africa and South Asia.
Currently some 18,000 children under five die every day. Half of those deaths take place in five countries: China, the Democratic Republic of the Congo, India, Nigeria, and Pakistan.
The immediate causes are mostly pneumonia, prematurity, birth asphyxia, diarrhoea and malaria, according to the report.
But 45 percent can be linked to under-nutrition, it said.
The report also said that, despite the improvements, the effort to cut childhood mortality was falling short of the target set in the Millennium Development Goals -- bringing the rate down by two-thirds between 1990 and 2015.