Rory McHugh's flicked goal in added time proved to the be the matchwinning score in a keenly contested game.
This was the third meeting of the teams this season, with London having relegated Sligo from Division 2B of the Allianz League last month.
Sligo led by 1-10 to 0-12 at the interval that's to Diarmuid Hannify's deflected three-pointer - although London had 10 wides in the opening 35 minutes.
London got back on level terms in the second half but Sligo persevered and managed to edge in fron again - and the McHugh in added time was the critical score.
Sligo are in a six-team group for the Christy Ring Cup with Tyrone, Derry Meath and Mayo.
The top two counties will qualify for the competition's final after the round-robin series.
Christy Ring Cup results
Sligo 2-20 London 1-20
