I would at least put road tires on your mountain bike - they really improve your efficiency on pavement.
If your commute is urban w/ lots of stops & traffic navigation, the mountain bike handlebar setup can be more maneuverable for tight situations than a road/touring bike.
I do miss the maneuverability of my mountain bike since switching to a road bike.
A commuter or hybrid bike may be a very good option, with the handlebar & fork designed for more urban situation vs. being tuned for long-haul riding.
bicycles.stackexchange.com/users/1616/Michael Ekstrand
My advise, buy a suitable commuter bike and keep the mountain bike if you have room.
I was in the same situation living in NYC.
I found that conversion was not worth the effort and simply switched to a commuter bike.
Converting your mountain bike would help but still leave you with some of the drawbacks, such as a heavier frame and shock absorption, which drains your efficiency.
A commuter bike will surly get you out on the road more.
Your local bike shop will help you find the perfect bike.
Things to keep in mind:
Frame size is keyCan
you add a rack/
fenders?Do you want a single speed or multiple (i suggest multi-speed)
bicycles.stackexchange.com/users/2630/J Gomez
Funny you're in Seattle, I had the same experience when I was out there for a summer.
I just got "slicks" for tires and it was fine.
bicycles.stackexchange.com/users/2359/kekekela
If you are commuting daily for a long distance, I'll switch to a road bike.
If not and you like your current bike, keep it!
I own a road bike and a mountain bike, depending where I'm heading.
But I do not live in a large city.
bicycles.stackexchange.com/users/2652/Gabriel Mongeon
I have always ridden mountain bikes, and even now that I commute daily since one year ago, most the time I go with my mountain bike.
For sure, if you ride in the city and not on off-road trails, chosing a thin lightly threaded tire is the best thing to do.
I don't like too thin a tire for 26 inch wheels, though, because the ride becomes very harsh with them.
I now also have a fixie, it is SO MUCH faster and easier to go around, but it seems very obvious to me that a skinny-tire-much-lighter bike without fenders and racks and all should be much easier to ride anyway.
bicycles.stackexchange.com/users/2355/heltonbiker
I recommend keeping a commuter bike for road use and a mountain bike for offroad use.
You can always ride a mountain bike on the road if you need to (albeit less efficiently than your commuter bike).
A nice, durable steel-framed hybrid is what I use for commuting, though that's really just a matter of prefence.
I bought a $350 mountain bike several years ago, and I learned the hard way why cable locks suck.
If you're going to lock your bike up in a public place invest in a good u-lock or something better than a cable lock.
Also, replace quick-releases with bolt-ons as those same douchebags also like to steal wheels and seats.
bicycles.stackexchange.com/users/2596/krs1
Start with your current bike, switching to smooth tires.
If you find your current bike comfortable, stick with that.
Only when you find yourself putting in real mileage (let's say 15 miles/day), then get a road bike (or a fixie if that's your bent) to improve the efficiency.
bicycles.stackexchange.com/users/174/Trey Jackson
There is nothing you need to do, but you'll probably find that the riding is more pleasant with road/commuter tires – ones with minimal tread.
You may also find that a mirror makes life feel saner and safer.
After that, you may find that the gearing for a mountain bike is lower than you'd like.
If you do, you can change the cassette (rear gears) to get a higher range without too much cost (maybe US $20-40).
Or, if you'd like to do some trail riding as well, you might do well to get a second set of wheels.
That would enable you to switch easily between knobby and slick tires and also have cassettes that are better suited to each style of riding.
bicycles.stackexchange.com/users/19416/dlu
I live in NYC (Bronx) and while there are a quite a few bike lanes and paths all there aren't many bike trails that warrant owning a mountain bike.
I see a lot of people by me who buy full suspension mountain bikes, but the majority of the time they ride in on the streets, which is such a waste of efficiency and money.
yet again I see a lot of people who think they are Lance Armstrong and buy $2K+ road bikes and ride them like idiots.
I would get a hybrid or a cyclocross which can handle some potholes and minor off road terrain while still keeping efficiency up.
I personally ride a Kestrel RT-1000 flat bar, which is basically a road bike that kestrel uses a flat bar on.
This makes it fast, efficient, but with a more upright position that's nimble in stop and go traffic.
bicycles.stackexchange.com/users/37904/Joseph Park
I just moved from where there wasn't much of a biking system infrastructure.  There were lots of trails though, so I bought an OK mountain bike ($350).  Now, I've moved to downtown Seattle and everyone bikes.  I'd love to get into it but not sure if a mountain bike is efficient for road-use.  Should I look into a conversion kit?  Should I just get a road bike and have one off-road and one on-road?
Riding a Mountain Bike in the city
bicycles.stackexchange.com
<mountain-bike><city>
bicycles.stackexchange.com/questions/6559
bicycles.stackexchange.com/users/2635/Chad Moran
It is best not to use a mountain bike for road riding. The easiest option is to use the bike but change the tire for road tires. However, it is probably better to have a road bike for commuting and a mountain bike for off-road. If you can only afford one bike or only have room for one a hybrid or a cyclocross is a good option. Mountain bikes have the benefit of maneuverability , but the heavier tyres and frame can make it less efficient for road use, while the gearing may not be optimal for commuting. Changing to road tyres will add significant efficiency, but if you are cycling longer distances then switching to a commuter, hybrid or road bike may be the best option.
Mountain bikes have the benefit of maneuverability , but the heavier tyres and frame can make it less efficient for road use, while the gearing may not be optimal for commuting. The biggest single difference will come from changing to road tyres, but if you are cycling longer distances then switching to a commuter, hybrid or road bike may be the best option.