Leicester Tigers players Croft and Ellis watched from the bench as the Lions made it six wins out of six in the final warm-up game before Saturday’s first Test in Durban.
The Lions scored tries through Ugo Monye and a penalty try, while replacement fly-half Ronan O’Gara landed two penalties and two conversions.
Former Leicester Tigers favourite Jaco van der Westhuyzen had kicked the Southern Kings into the lead with an early penalty.
The Southern Kings, for whom former Tigers second row Marco Wentzel had accepted an invitation to play, added a late try through Mpho Mbiyozo to complete the scoring.
British & Irish Lions: Keith Earls, Ugo Monye, Gordon D’Arcy, Riki Flutey, Luke Fitzgerald, James Hook, Mike Blair; Andrew Sheridan, Ross Ford, Euan Murray, Simon Shaw, Donncha O’Callaghan, Nathan Hines, Joe Worsley, Andy Powell
Replacements: Matthew Rees (for Ford, 64), Adam Jones (for Murray, 7), Tom Croft, David Wallace, Harry Ellis, Ronan O'Gara (for Hook, 12), Shane Williams (for Monye, 64)