The biggest surprise came in a Western semi-final, where the Seattle Sounders pounded FC Dallas, the best MLS team in the regular season, 3-0.
In Montreal, 31-year-old Mancosu outraced two defenders and one-timed a screamer just inside the box to the left of goalkeeper Luis Robles in the 61st minute.
The Italian forward has six goals and five assists in 16 appearances since arriving in July on loan from Bologna in the Italian Serie A league, including two goals and an assist in the 4-2 win over DC United in the knockout round on Thursday.
Mancosu's role as a scoring target as increased since the departure of striker Didier Drogba, who has 10 goals but hasn't played in a month.
In the other Eastern semi first leg, Toronto FC scored two late goals to beat New York City FC 2-0.
United States striker Jozy Altidore scored in the 84th minute and Canadian midfielder Tosaint Ricketts added a goal in stoppage time.
Toronto dominated play in the second half but could not finish until Altidore latched onto a loose ball and blasted home from close range.
Ricketts, just into the game as a substitute, also scored from close range.
There was some pushing and shoving at the final whistle, with Altidore among those involved.
In California, Mexican Giovani dos Santos scored in the 55th minute to give the Los Angeles Galaxy a 1-0 win over the Colorado Rapids.
In the other Western semi, the Sounders produced their best eight-minute stretch in playoff history.
Paraguay forward Nelson Valdez scored the decisive goal for the second straight match after not putting one in the back of the net all season. He connected on a cross in the 50th minute.
Uruguay midfielder Nicolas quickly added two more goals for Seattle, leaving Dallas on the brink of elimination heading into the return leg.
(Editing by Andrew Both)
