I am trying to put on muscle mass and get cut at the same time.
Every time I do cardio, I lose muscle mass. I am 6'1" 245 and I just can't get more muscle mass while doing cardio. Am I doing something wrong or can I not have the best of both worlds?

It is very difficult to add lean body mass if you are doing a higher level of cardio or if you are on a tight diet. Why are you doing cardio? If you are trying to gain mass you should not be doing cardio, or very little of it. If you are trying to get ripped, then you should be doing some cardio. But, the focus that you must have during these times is to maintain your lean body mass while decreasing body fat.

Ultimately, you should break down your entire training year. There should be times when you focus on gaining mass. These mass gaining phases should be 12 weeks in length. Followed by 1 month of a leaning out phase where you can do 30 minutes of cardio 5 days per week. During the mass building phases don't do cardio for more than 20 minutes 3 times per week. Also, if you weigh 245 pounds you will not want to go below approximately 3000 calories during your mass building phase. Keep your protein intake at one gram per pound of bodyweight as a minimum.

Once you have been through two 12 week mass building phases with the shorter one month leaning out phases you should go on a 12 week cutting phase. This cutting phase will take you from a peak mass phase to a cutting phase throughout a 12 week program.

Here is a response that details what to do to maintain or slightly increase your lean mass while your transition into a bodyfat reduction phase.
The response below has been individualized for a person that weighs 215 pounds. However, all of the information can be converted to your individualized needs by using your own bodyweight in the calculations. Let me know how it goes!

You need to be doing either the 5 Day Bodybuilding Mass Program or the 5 Day Strength and Power Program. You can choose the EXPRESS or the Advanced EXPRESS version of the Strength and Power program if you would like. Then, you need to diet! To start with, take in at least 1 gram of protein per pound of body weight. This would be 215 grams of protein in your case. Then, to find the number of carbohydrate grams you should take in, just multiply your bodyweight by 2. This would be 430 grams of carbohydrates in your case. Finally, you should take in 72 grams of quality fatty acids like Flax seed oil, Olive oil, or Udo's oil. This is found by dividing your bodyweight by 3. Check out the recommended readings in the fitness info section of our web page to find out about Udo's oil! I have done some calculations for you but I also want to explain what is going on here. Here goes.

1 gram of protein has 4 calories. 1 gram of carbohydrate also has 4 calories. 1 gram of fat has 9 calories. For you this means.

First Week of Program:
Protein (215 grams x 4 = 900 calories)
Carbohydrates (430 grams x 4 = 1720 calories)
Fat (70 grams x 9 = 630 calories)

First Week Total - 3250 calories.

When you start your program you should start with the 3,250 calories with the appropriate balance of protein, carbs and fat that I have outlined above. But, here's the kicker! Each week of the 12 week program you should add 10 grams of protein and subtract 15 grams of carbohydrates to your diet. But, keep the fat intake the same.

So, you should end up on the last week of your 12 week program with these totals:

Protein (325 grams total)
Carbohydrates (250 grams total)
Fat (same at 72 grams total)

This would give you a new caloric range of:

Protein (325 x 4 = 1300 calories)
Carbohydrates (250 x 4 = 1000 calories)
Fat (70 x 9 = 630 calories)

Total on week 12 of your program - 2030 calories

Make sure you spread out your eating to 6 times per day so you are always bringing in some nutrients into your body for growth and recovery without eating so much that you store any extra calories from fat.

You will also need to kick in some cardio training to really get lean. But, not too much so that you start to lose your mass. On weeks 1-4 you should do 25 minutes of cardio, 3 times per week. On weeks 5-9 you should do 30 minutes of cardio 4 times per week. On weeks 10-12 you should do 30 minutes of cardio 5 days per week. Keep your heart rate between 130 - 140 beats per minute or 70% of your age predicted maximum heart rate (APMHR). You can find out how to get your APMHR by typing these letters into the Search function on the Fitrex home page.

Finally, you will see a drop in bodyweight with this type of diet plan. However, If you eat the way I have told you to, and if you are mentally strong enough to push yourself with your training you will keep the mass that you have built and the weight loss will come from fat loss! Since everyone is different you may have to adjust your carbs and fat intake down slightly but don't reduce the protein level. However, I followed this very same plan (with adjustments for my specific body size) and I lost 36 pounds of fat in the first 8 weeks of the plan! Give it a shot and let me know how it goes.