The decision to make the combined post redundant follows a structural review of the Cunard PR and social media team at Carnival UK HQ in Southampton led by director of PR and social media, Rachel Lloyd.
Lloyd joined Cunard last spring. Soon after her arrival, and for the first time since 1997, Cunard appointed an external PR agency.
Gallagher joined Cunard in 1994.
He has overseen numerous significant milestones in Cunard's recent history including the build, inaugural programmes and entry into service of the current three Cunard Queens.
He also played a pivotal role in events and celebrations to mark 2015's 175th anniversary of Cunard's first transatlantic crossing from Liverpool to Boston.
A prolific author, his knowledge of Cunard history is regarded by maritime experts around the world as encyclopaedic.
Gallagher has also been instrumental in managing Cunard's relations with Buckingham Palace and has planned more than 20 Royal Visits to the fleet, including to the most successful Cunarder in history (Queen Elizabeth 2) and the Royal naming ceremonies for Queen Mary 2 (HM Queen Elizabeth), Queen Victoria (HRH The Duchess of Cornwall) and Queen Elizabeth (HM Queen Elizabeth).
Gallagher declined to comment on his departure from Cunard.
A Cunard spokesman told Seatrade Cruise News: 'The role of company historian exists and will continue to do so. Cunard is incredibly proud of its heritage and we look forward to ensuring it continues to be celebrated for the next 178 years and beyond.'