Reporting to new chief executive Jon Lewis, Hall will take charge of all internal and external communications, including employee engagement and public affairs, at the IT and administration group.
Hall left HSBC last year to join board-level mentoring organisation CMi (Chairman Mentors International).
While working at the bank as group head of external affairs, she oversaw comms, public affairs and sustainability.
Before joining HSBC, Hall was deputy director-general of the Confederation of British Industry (CBi), where she led its public policy development and advocacy work in the UK and overseas. She has also worked in human resources at the BBC and is a non-executive director at the Office for Students.
Lewis said: "Effective communication is critical to Capita’s future success. I am pleased to welcome Katja to Capita at an important time in our transformation and look forward to working with her to engage all our stakeholders on the progress we are making on this journey."
Hall said: "I look forward to working with Jon and colleagues on the ongoing transformation of the business, building strong relationships with our public and private sector clients, including key political stakeholders; and, in ensuring our employees understand and engage with the changes our business is going through."
Since Lewis’s arrival in December 2017, Capita has raised £662m in a rights issue and disposed of £416m in non-core assets.