Ammar bin Yasir acted as governor for Umar Farooq
Tarikh al-Islam” vol. 2 p. 581
Jawad Chirri, a Shiite scholar, writes in his book, Imam Ali
People of Kufah knew Ammar Ibn Yasir and his brilliant record in Islam. He was the governor of their city during the reign of Omar.
We read in Isaba and Istiab that Umar wrote to the people of Kufah
فإني بعثت إليكم عمار بن ياسر أميرا وابن مسعود معلما ووزيرا، وإنهما لمن النجباء من أصحاب محمد صلى الله عليه وسلم، من أهل بدر، فاطيعوا لهما واقتدوا بهما
I am sending towards you, Ammar as Amir and Ibn Masud as teacher and minister, and they are amongst the noble companions of the Prophet (s), hence obey them and follow them.
Istiab 4/80 , Isaba 2/369