Mississauga covers 288.42 kmē of land fronting a 13km shoreline on Lake Ontario.

Mississauga is bound by Oakville and Milton to the west, Brampton to the north, Toronto to the east, and Lake Ontario to the south. Halton Hills borders Mississauga's north-west corner. With the exception of the southern half of the border with Toronto (Etobicoke Creek), Mississauga shares a land border with all previous mentioned municipalities.

Two major river valleys feed into the lake. The Credit River is by far the longest with the heaviest flow, it divides the western side of Mississauga from the central/eastern portions and enters the lake at the Port Credit harbour. This indented, mostly forested valley was inhabited by native peoples long before European exploration of the area. The valley is protected and maintained by the Credit Valley Conservation Authority (CVCA).

Etobicoke Creek forms part of the eastern border of Misssissauga with the city of Toronto. North of there is passes through Pearson Airport, where there have been two infamous aviation accidents where aircraft slid into its banks. This valley experienced some heavy flooding with some homes along its edge being swept into the lake as a result of heavy rains from Hurricane Hazel. This creek and its tributaries are administered by the Toronto and Region Conservation Authority (TRCA).

Most land in Mississauga drains to either of the two main river systems, with the expection of the smaller Mary Fix Creek which runs roughly through the center of Mississauga entering the lake just east of Port Credit. Some small streams and resevoirs are part of the Sixteen Mile Creek system in the far north-west corner of the city.

The post-glacial Iroquois Shoreline roughly follows the Dundas Street alignment, although it is not noticeable in some places and in others the shoreline affords a view of downtown Toronto. The land in Mississauga in general slopes gradually downward from almost 190m ASL in some northern spots to lake level (76m ASL), a 110m difference over an averaged 15km distance.