I have recently been experiencing pain in my left knee on the outside only, it doesn't seem to be in the joint but rather on the outside of the joint. Starts to hurt after 20 miles or so, never off the bike, and ends after only a few minutes off the bike. It is a really sharp solid pain, but only when pedaling. Any ideas?

This is a frequent topic on this forum so you will get good advise with the search function. My personal experience with this was saddle height adjustment. Generally for me, higher is better. Of course assuming high cadence pedaling of 80 to over 90 RPM.
If that does not do it, you will need a Doctor.

I do pedal at a fairly high cadence, and ride clipless, thanks for the info. Hows Rockford, used to live just off of Alpine on Harrison.

Rockford is booming with shopping, motels, hospitals and restaurants. The east side has grown like mad. That is where we live on Roxbury NE of St. Anthony. I think we are becoming a retirement alternate for Chicago.
Please be careful with your knee. Have you changed saddles lately? My wife had knee problems until we got the saddle height right. Ditto me.

It is also possible that you need a wedge on your shoe or that one leg is a bit longer than another. Maybe a slight rotation of the cleat would help as well.... There is a lot of discussion on this topic availble in past posts.

You can also try strengthening your abductors, adductors, vastus medialus, and
hamstrings.

I suspect iliotibial band syndrome. Google it. The cure is to strenghten the muscles that hold the knee in place. That would be the abductors and adductors. You can do this at home, but my personal experience is that a gym with an abductor/adductor machine works several times faster.

Now, having said that, the fact that it is solely on your left could be telling us that one leg is shorter and the left has wound up trying to correct for the imbalance. Take a gallon milk jug and cut the front off. Place your innersole on it and cut out a section the size of the innersole ahead of your arch. Tape that to the underside of the innersole of your shorter leg. The get one Lemond LeWedge and place that so that the thick part is to the outside. You may have to experiment to find the number and orientation of the wedges you need.

Another trick is to adjust the cleats. Slide the cleat to the inside on the short leg and to the outside on the long leg.

One last trick is to use different innersoles. Use thin ones on your long leg and thicker ones on the short one. This one is a Hail Mary.

I find a number of small tweaks is better than one big one.

I didn't mention that my left foot also go's numb but not the right. Related?

Very slight adjustments (2-3 mm's) may correct the problem. Generally when there is pain in the front of the knee, you slide the seat back away from the bars a little. I had the front of knee pain issue in one knee recently. I opted to move the cleat forward a few mm's on that shoe only as the other knee was fine and it worked for me.

Do your cleats have some float by chance? Have you been fitted to your bike or have you tried to use your measurements to try and get close?

What do you mean by float? Slop? I rode toe clips for years, went to clipless last year. Had foot numbness all last year but no knee pain. I helped the numb issue to some extent with seat adjustments.

Cleats that force your foot to rotate slightly from its optimal position can definitely cause knee pain. Angular float is a good thing, and you may have had more of it with toeclips than with your current clipless system.

Pop on over to amazon.com or ebay.com and order yourself a copy of "Save Your Knees," by James Fox, M.D., the official orthopedic surgeon of the 1984 Olympics.

Just an update. I moved the cleat on my left foot back about 1/8" and rode 36 miles last night with no knee pain at all. Still a little numbness in the foot, but way better than before.

IT band syndrome, I'd say for sure. I've had that problem a couple of thousand times it seems. The best way I know to cure it is make sure the seat is high enough, and develop a stroke with the inside of your thighs brushing the top tube of the bike each revolution. Basically, you want your knees( and the pedalling pressure) to stay in a straight up and down line, not pushing off to either side at all. Whenever I feel a twinge on the outside of my knee, I concentrate on trying to pedal with my big toes flexed. That ensures that I am pushing with the inside of my foot.

Also do stretching. You should be able to stand on your right leg and with your left hand hold your left foot against your rear for 30 seconds. Repeat 3 times if any tightness. And repeat daily until you have flexibility.

Another option is to go see a physical therapist for relaxing the tendons if the stretch does not work.

Don't know if IT Band Syndrome is the same as Patella-Femoral Pain Syndrome. In my case that was caused by a complicated sequence from upper glutes to IT band tension. With physical therapy it was easy to solve (again, in my case). In fact the day after day 1 of PT there was no pain.

Edit: Forgot: original cause was leg length discrepancy of .5 inch. Compensation cascaded thru a bunch of things ending in Patella-Femoral Pain Syndrome.

This wasn't difficult to deal with. Still, hope that you don't have it.