I've played the first game extensively in the past and it's not too hard to make a Fighter-Mage, though you do sacrifice a lot of points in order to do this. I enjoyed the idea of playing a character who could excel at both spells and swordplay at once.

My question is about the later games. First: are you required to take your shield with you to Raseir in Trial by Fire? I played the VGA version created by AGD. I can't remember exactly because it's been more than a year since I've beaten Trial by Fire but I want to say I left my shield in my room at the inn, but took it with me when I left with the caravan. This turned out to be a mistake for me since I had been using a ton of magic in combat and I had to fight Khaveen the old-fashioned way. I'd just like to know if you can leave it in your room before you leave for Raseir.

Secondly, how does this work in the later games? Can you use magic in combat as a Fighter or are you required to just use your sword?

In QfG2, you could (almost) always "drop shield" to be able to cast spells while fighting, though for Khaveen if you don't have a shield and try to fight him, you lose at once. So you'll either have to bypass him the mage's way, or fight him with a shield. Also, if you do have reversal, you will have the mage's end fight (at least I hope reversal is the cause, if it's magic skill instead, you are probably screwed).
In QfG3, you can't cast spells in combat, though nothing prevents you from bombarding the enemy with flame darts while he approaches.
In QfG4, you can cast spells in close combat even while fighting with sword and shield.
In QfG5, you can't cast most spells if you are holding a large weapon, though you can always zap your weapon, or remove it to cast the other spells, and then re-equip it.

Thanks for the clarification.

I did have Reversal and I still fought Khaveen so maybe it's tied to your actual class, though when I import that character into QFG3 it says I'm a Wizard (though I change it to Paladin since I was able to meet all the requirements at the end of 2).

Shame about not being able to use magic in combat in 3, does that go for Wizards as well?

If your wizard has a shield, he won't be able to cast in combat, if not he can freely cast.

If you play 3 as a wizard, whether by creating the character in 3 or by importing it from an older game, you won't have a shield, nor can you acquire one.