 Since 1962 Mühlen taught at the Divinity Faculty of the University Paderborn, where he later (1964–1997) worked as "Ordinarius für Dogmatik und Dogmengeschichte" (Professor of Dogmatics and Dogmatical History).During the Second Vatican Council, Pope Paul VI appointed Mühlen as one of the theological experts (1964).His theological work is concentrated mostly on pneumatology, ecclesiology and pastoral theology.