The 27-year-old frontman latched on to a through ball to lash home past goalkeeper Shay Given to bring his side level in the friendly at the City of Manchester Stadium.
Killen, who now has scored five goals in six pre-season friendlies for Celtic, proved a handful for the Manchester City defence and went close on two other occasions.
He also found the back of the net against Tottenham Hotspur, Brisbane Roar and Egyptian side Al Ahly.
Celtic begin their Scottish Premier League campaign on Saturday against Aberdeen.