In a wide-ranging address at the Myanmar Institute of Strategic and Studies in Yangon on May 27, Mr Mitchell said he wanted to clarify the decision because much of the attention it had received in Myanmar was exaggerated and based on misunderstanding.
He was referring to President Obama's May 15 decision to extend some sanctions under the National Emergencies Act, which prohibits American businesses and individuals from investing in some Myanmar entities or doing business with some Myanmar citizens.
Mr Obama said that despite the progress Myanmar has achieved in its reform process "the situation in the country continues to pose an unusual or extraordinary threat to the national security and foreign policy of the United States".
Acknowledging that the issue was "a bit technical", Mr Mitchell said the annual declaration of a national emergency since 2011 had both renewed certain restrictions and granted the president the ability to authorise other actions to ease restrictions.
The latter had included waiving restrictions on the export of financial services and on new investment by US companies, he said.
Mr Mitchell said the decision kept in place sanctions "on those individuals and entities that materially benefited from their close ties to the former regime and who are still impeding reform in this country, the so-called 'specially designated nationals' or SDNs".
But even they are able to have their cases reconsidered if they can demonstrate changed behaviour and support for reform, he said.
"We hope these individuals can eventually become responsible stakeholders in this country's political and economic transition and bring benefits to the people of Myanmar," the US ambassador said.
Far from being a step backward, said Mr Mitchell, "the renewal of the national emergency has allowed us to continue to encourage the positive reforms underway, keeping in place our ability to licence activities ? including encouraging responsible investment by US companies among other actions ? that will provide material benefits to the people of this country and support reform".
Referring to the reform process, Mr Mitchell said it remained a work in progress.
"Because of the legacy of the past, questions abound among citizens of this country and my country about the future course of reform," he said.
There were questions about the role of the military and whether "this proud and important institution will evolve into a professional force trusted by and at peace with the Myanmar people...", and whether personal and civic freedoms, including those of association, assembly, media and speech, would be protected and institutionalised in law and applied to all.
"They also have serious questions about Rakhine State, a state with so much promise and yet one that suffers from chronic underdevelopment, internal strife and now a humanitarian crisis that is receiving growing international attention," Mr Mitchell said.
He said it was essential that the government recognised the questions that remain about the reform agenda and that it continued to demonstrate momentum in the reform process "to reassure its citizens and international observers alike about the credibility, sustainability and institutionalisation of reform."