John Hannett took control at shopworkers' union Usdaw on Monday, and pledged to build upon departing general secretary Sir Bill Connor's legacy.
Hannett will continue building Usdaw's membership and representation, and vowed to maintain the union's support for a third term in Government for Labour.
Hannett, 50, has been an Usdaw member since he was 17, and became deputy general secretary in 1997.
He said: 'Retail is an ever-changing industry. We have to keep up with the changes, anticipate them where possible and respond accordingly. We are making progress, and that's down to our army of reps working to recruit and organise.'
Usdaw, with 330,000 members, is the UK's fifth biggest union, and has made its voice heard on issues such as Christmas Day trading and violence against shop staff.
Connor marked his departure with an open letter urging trade unionists to back Labour.