She will report to CEO Nathan Bostock, and be responsible for implementing the bank's external and internal comms strategy, as well as leading on public affairs.
Shield is currently at FTSE 100 insurer RSA, where she has been director of comms since 2010.
Before this she had been Lloyd's of London's head of comms. She worked in Government and then at the Department of Education and Skills, rising to head of newsdesk, between 1999 and 2003.
Jennifer Scardino, head of international commms at Santander Group, retains her global role, and the pair will work together and both sit on the UK executive committee.
Bostock said the new hire would "help us further develop our solid reputation and enhance our position as a full-service challenger bank". A study last month named Santander as having the third best reputation of the 12 biggest banks in the UK.
Shield said: "I'm thrilled to be joining Santander. It is recognised for its excellence in customer service, innovation and creating real benefit for the UK economy and society. I am proud to be joining the bank to help maintain and enhance that reputation."