London, who finished second to Rovers in the Championship's regular season, opened the scoring through Andy Ackers.
Danny Addy, Kieren Moss, Robbie Mulhern and Ryan Shaw then went over to put the hosts 25-8 up at the break.
A second from Shaw sealed victory, before London finished strong with tries from James Cunningham, Alex Walker, Mark Ioane and Elliot Kear.
The win ensures KR maintain their perfect start to the Qualifiers and puts them equal on points with leaders Warrington Wolves after three matches.
Hull KR: Moss; Shaw, Hefernan, Blair, Carney; Atkin, Ellis; Jewitt, Lawler, Masoe, Clarkson, Addy, Kavanagh.
Replacements: Lunt, Greenwood, Marsh, Mulhern.
London Broncos: Walker; Williams, Hellewell, Kear, Dixon; Sammut, Barthau; Spencer, Ackers, Ioane, Wilde, Pitts, Evans.
Replacements: Cunningham, Davis, Gee, Roqica.
Referee: Chris Campbell.
Hull KR survived a London Broncos fightback to boost hopes of making an immediate Super League return.