Stanley, 30, was a Scouts volunteer for five years, beginning in 2009. He is also a former City of Ottawa lifeguard and swim coach.
He pleaded guilty on Thursday to 10 counts of sexual interference, three counts of luring and three counts of invitation to sexual touching.
The boys were between the ages of 12 and 15 at the time of the abuse.
The molestation began in the summer of 2012 and continued for a year and a half, according to an agreed statement of facts read in court. Stanley took the boys to movies, hosted visits in his parents' basement and drove the boys to secluded locations, court heard.
Stanley told one boy not to tell anyone about the sexual abuse because he would get in trouble, court heard.
Another Scouts leader told CBC News that he warned the organization that Stanley was spending time alone with boys, despite the "two-deep rule" that requires scouts to always be under the supervision of at least two adults. He said some of the abuse could have been prevented had the organization heeded his warnings sooner.
Scouts Canada did not comment Thursday but has scheduled a news conference for 10 a.m. Friday
Stanley is expected to be sentenced on Sept. 25.