Mass Effect Legendary Edition has a mouse acceleration problem. Players are logging into the game on Steam and finding that Mass Effect 1 suffers from a bug that causes players' mouse acceleration to spike. With this bug, the game becomes just about unplayable with the mouse and keyboard. Here's how to deal with this bug.
Mass Effect Legendary Edition Mouse Acceleration: How to Fix
EA and BioWare have yet to release a fix for the mouse acceleration bug, so players who want to play with the mouse and keyboard are on their own in terms of fixing it. There are a few community-built solutions so far.
One is to download and implement a player-made file for the original Mass Effect 1 PC port that removes the game's mouse acceleration. It can be installed by simply extracting the one file into your game's install directory. Find it and installation instructions here.
Another possible fix is a little more involved and requires modifying the game's install files. You can find the steps to doing so here.
Of course, if none of the above appeals to you or works for you, you may want to just swap to a controller and circumvent the problem entirely.