He won a fellowship to All Souls College, Oxford, and taught there and at Christ Church where his pupils included A. L. Rowse.
He was an able academic politician, and is said to have recruited Sir Lewis Namier to Manchester by reading in his newspaper that Namier had no position, making a phone call to invite him to take a chair, and only then walking over to tell the vice-chancellor of the recruitment.
[2] Jacob is remembered as the link between the old school of 'structuralist' medievalists, including distinguished names such as William Stubbs, T. F. Tout and F. W. Maitland, and the subsequent school of more socio-political medieval historiography, to which J. S. Roskell, K.B.
His professorships at Manchester and Oxford did much to make the two schools England's academic centres for medieval studies.
He also contributed the Fifteenth Century volume to the landmark Oxford History of England series.