First of all, you have to understand that there really was no real German language until at least the 15th century. The German language consisted of what is called a dialect continuum, meaning that there was no defined version, rather several different dialects which differed slightly from town to town, until, after several towns, people could barely understand each other.

Moreover, the Anglo-Saxons who emigrated to what it now called England, came from Northern Germany. Even before those settlers left their homelands, the German dialects further south had undergone the High German consonant shift which left a massive impact on the language. Northern Germany and the Netherlands were not affected by this, meaning that Old High German, which would develop into modern German, and Old Saxon, which would evolve into English, differed a lot even before the settlers left their home. Differences between German and English like Apfel<>apple, Silber<>silver or machen<>make are a result of this consonant shift.

To put into perspective, Old Saxon would evolve into Low German in Northern Germany, a language which isn't mutually intelligible with High German even now. To sum it up, High German and English were never mutually intelligible. If instead you would like to know when Low German and English speaker couldn't understand each other anymore, now that is a very different question.

edit: grammar