Federica Mogherini was in Lausanne, Switzerland last week, where she met with US secretary of state John Kerry and Iranian foreign minister Mohammad Javad Zarif. The Chinese, French, German, Russian and British foreign ministers were also present.
The officials met to discuss Iran's nuclear programme, which has been a source of conflict with the west since its existence was revealed in 2003. Amid fears that it was developing nuclear weapons, Iran has been hit with sanctions over the years, such as bans on crude oil exports and heavy weaponry.
The US and the EU have now agreed to lift these sanctions, in exchange for Iran pursuing a peaceful nuclear programme.
A statement from the participants of the negotiations explained, "Iran's enrichment capacity, enrichment level and stockpile will be limited for specified durations and there will be no other enrichment facility than Natanz. Iran's research and development on centrifuges will be carried out on a scope and schedule that has been mutually agreed".
While this agreement has been praised by the international community, it will not be finalised until 30 June, when it is hoped a joint comprehensive plan of action will be completed.
"The technical details of this political agreement must now be successfully ironed out before the June deadline" - Martin Schulz
Mogherini said "a good deal" had been reached, and was thanked for her efforts by European climate action and energy commissioner Miguel Arias Cañete, who stressed that these recent developments were "good news for global peace, energy safety and security - implementation is now crucial".
The deal was also cautiously welcomed by European neighbourhood policy and enlargement negotiations commissioner Johannes Hahn, who congratulated the EU foreign affairs chief on reaching a "basic agreement", adding, "I hope it is sustainable".
European parliament president Martin Schulz said, "I salute the historic deal", highlighting that "this landmark agreement is a major contribution to internal security, increases mutual confidence and will bring relief to the economy of Iran, to the benefit of its people".
Yet Schulz also pointed out that "the technical details of this political agreement must now be successfully ironed out before the June deadline".
Janusz Lewandowski, chair of parliament's delegation for relations with Iran, said the deal means that "there is hope for a breakthrough in relations" with Tehran.
Calling the outcome of the talks "a major diplomatic success", S&D group president Gianni Pittella explained that they "demonstrate the effectiveness and importance of a genuine European role on the international scene".
However, Socialist group vice-president Victor Boştinaru warned that "there is still a long way to go until the deal is finalised", though he did concede that it was "a strong and positive signal that the negotiators of the deal were able to set aside decades of distrust in order to come to this agreement".
ALDE group president Guy Verhofstadt commended Mogherini's "great leadership of EU diplomacy".
Marietje Schaake, a substitute member of parliament's delegation for relations with Iran, pointed out that "hard work remains to be done in order to work out details that should lead to mutual trust that Iran will not pursue the development of a nuclear weapon, and that Europe, the US and the international community will lift sanctions".
In her view, "Europe must seize this moment to work on other important issues on our agenda, such as countering terrorism in the Middle-East and the human rights of the Iranian people".