How to fix negative canthal tilt
First check whether yours is real. Most photographed ones are not.
Read this first
The angle cannot be changed without surgery. It is set by where the lateral canthal tendon anchors to the orbital rim, and no exercise, massage, routine or supplement moves a tendon insertion. Anything sold as doing so is not doing so. What is worth your time instead is finding out whether the tilt you measured is real, because most negative readings from phone photographs are the camera.
There is no non-surgical way to change canthal tilt, because the angle is determined by where the lateral canthal tendon attaches to the orbital rim. What can be changed is how the eye area reads and whether the measurement was real in the first place. Start there: a phone held below eye level at arm's length applies enough perspective distortion to flatten or reverse the apparent tilt, which is why a mirror and a selfie so often disagree, and why most negative readings people arrive with are the photograph. Re-measure at eye level, as far from your face as your arm allows, in flat frontal light. If the tilt is genuinely negative it is still common, appears in every population, and is a description of an angle rather than a defect. What does change the appearance is under-eye volume, brow position and sleep.
What actually works
Re-measure at eye level
Camera at the height of your own eyes, as far back as your arm reaches, head level, flat light from in front. This single change reverses the sign of the reading for a lot of people.
Rule out puffiness
Under-eye fluid lifts the lower lid and changes where the outer corner appears to sit. Measure in the evening as well as the morning; if the two disagree by more than a degree or two you are measuring sleep, not anatomy.
Check the brow, not the eye
A low or flat brow tail makes a neutral tilt read as negative. That is the part that responds to grooming, and it changes the impression without touching the measurement.
Stop if the answer is surgical
Canthoplasty and canthopexy are the procedures that change this angle, and they are surgery on the eyelid. That is a decision for a surgeon, not for a website, and this one has nothing further to offer on it.
Measure it, so you know whether anything moved
What is my canthal tilt?The inner-to-outer eye corner angle, in degrees, per eye.
Questions
Can exercises fix negative canthal tilt?
No. There is no muscle whose training moves a tendon's bony insertion, and the exercises sold for this train the orbicularis, which is not what determines the angle. Nothing in this category has evidence behind it.
Why does my tilt look different in the mirror?
Because a mirror has no lens. A phone camera at arm's length below eye level introduces perspective distortion that flattens the outer corners; the mirror shows you the angle without it, which is usually the more accurate of the two.
Is negative canthal tilt actually bad?
It is a description of an angle that appears across every population. The community treats it as a problem because a positive tilt is more common, but common is not the same as better, and no measurement here says otherwise.