The Catholic Church made great progress in Newfoundland during his episcopate of Mullock, a new diocese, Harbour Grace, being erected.

He also opened in 1857 St Bonaventure's, a school for middle-class boys, a new episcopal palace and library, eleven convents, and numerous churches.

When, in 1852, the Colonial Office refused to grant responsible government to Newfoundland, Mullock denounced it in extreme terms in a published letter.

In the ensuing election and its aftermath he pursued a somewhat inflammatory course culminating in his putting the inhabitants of Cat's Cove under the Episcopal Ban for political actions which displeased him.

When Mullock died the Governor, Sir Anthony Musgrave, attended the Requiem Mass, all the flags flew at half-mast, and all the shops closed.