Spencer, 66, will follow in the footsteps of countrymen Fran Cotton and Bill Beaumont as the third English Lions tour manager in rugby's professional era.
And the former England captain, who toured New Zealand in 1971, will be joined by ex-Ireland winger Tom Grace, who is the new chairman of the British & Irish Lions board.
"It is a huge privilege and honour to represent the Lions as tour manager, 46 years after I did so as a player," said Spencer.
"My predecessor Andy Irvine did a fantastic job in Australia, and I am looking forward to following in his footsteps."
Grace, meanwhile, was with the Lions in 1974 - touring South Africa - and won 25 caps for Ireland.
"It is an honour to be appointed chairman of the British & Irish Lions," he said.
"The Lions represent the core values of our sport, and I am grateful for the opportunity to represent them as chairman.
"It is a privilege to take over from Gerald Davies, who has been a wonderful ambassador for the Lions as a player, tour manager and more recently as chairman."