Prettejohn, who was until recently a non-executive director of the Prudential Regulation Authority, will take on the role at the pension provider on 23 June.
He replaces Lord Norman Blackwell, who became chairman of LBG in April this year.
Blackwell says: “Nick has significant financial services experience, particularly in insurance where he has served as chief executive of Lloyd’s of London and Prudential UK and Europe as well as chairman of Brit Insurance.
“He will be a valuable addition to the group board and is ideally suited to carrying on the role of chairman of Scottish Widows.
“His appointment will allow me to step down as chairman of Scottish Widows on 23 June 2014 to concentrate on my role as chairman of the group.”