She's had the job since 2007.
As of 11:45p.m. EST on Sunday, she was leading the race 10,117 to 6,074.
The EMSB vote was a neck-and-neck battle early in the night, with Mancini pulling ahead only around 11p.m. EST.
The election campaign was mired in controversy. Many English voters were left off the voters' list. Others arrived at polling stations to discover a line-up of 100 people in front of them.
An election official told CBC News the final results will be announced on Monday.